These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Nicholas Kerensky was either a driven visionary or a Complete Monster, and there's plenty of evidence for both.
The same can also be said for Sara McEveredy and Clan Wolverine. Innocent victims of Nicholas' manipulations, a powder-keg waiting to go off or power-hungry monsters who merely were caught out before they could act?
Hanse Davion and Katrina Steiner are awesome leaders if you lived in the states that became the Federated Commonwealth. Not so much if you were in one of the three neighboring states that got curbstomped in the process.
Sun Tzu Liao: Scheming, opportunistic weasel of a ruler, or brilliant political tactician?
Arc Fatigue: When the BattleTech license transferred to FanPro a significant number of new material focused on the year 3067, during the end of FedCom Civil War and the start of Jihad. Another arc that has run too long is the Dark Age era, not helped when its development outside of the novels was put on hold until the Jihad arc was finished.
Author's Saving Throw: Catalyst Game Labs treatment of the Dark Age era, which in early 00' managed to piss off many Battletech fans for various reasons.
Broken Base: The Double Heat Sinks. Either you think that they unbalanced the game in favor of energy weapons or you think that they made the energy weapons viable. And whatever group you belong to, it is almost universally agreed that trying to fix Double Heat Sinks would cause more problems than it solves.
The Jihad and Dark Age both tend to cause this as well.
The Clans, either they are the symbol of everything that is wrong with BattleTech outside of the 3025/Succession Wars era, or their arrival is a point when BattleTech has grown the beard.
Chronologically speaking, first there's Stefan Amaris, the Usurper. He's the lovely bundle of joy who overthrew the Star League, executed everyone in the Star League Court, killed everyone in the Vatican before burning the place down, and a whole laundry list of deeds including genocide and sterilizing planets.
Katherine Steiner-Davion was arguably a fine politician when first presented, but a combination of author fiat and other factors led up to the revelation that she'd hired an assassin to kill her own mother, her brother, her other brother's girlfriend, and her own boyfriend. All for political gain. After the rest of humanity started catching on to her game, she became the focal point of a civil war that wrecked both of the nations she ruled, sold out the nation she had the most loyalty with to the Clans (in an attempt to kill her brother and some mercenary outfits that weren't on her team) and finally - after being deposed - escaped with her Clan allies and created a test tube baby with her rival brother's DNA and her own who would hopefully go on to cause more havok in the Inner Sphere at a later date.
Even her aforementioned son couldn't take it. He ends up killing her after she advocates murdering an ally, to protect her own position. Especially since maintaining her position may get him and his Clan destroyed.
An even earlier example is Star League General Amos Forlough, who helped conquer the Periphery during the Reunification War. While his actions against the Taurian Concordat, which included saturation level orbital bombardment and WMD deployment, may have been justified due to the fact that they were doing the same things; his actions on the Outworlds Alliance front went way too far. The Alliance barely had a regular military and so they used irregular militia to fight back with bombings and sniper attacks against his troops. He responded by killing 10% of the population of every planet that resisted (roughly 12 million people across the Outworlds), left many more without homes or shelter and sentenced his own subordinates to death and hard labor if they refused to carry out these atrocities. Little wonder why he is known as "The Butcher" and "The Baby Killer". The worst part is that he was never brought to justice and received some of the Star League's highest rewards for the victories he achieved.
Designated Hero: When you think about it, Hanse Davion got a few hundred million people killed to impress his hot eighteen year-old Nordic wife. And to get payback from the man who tried to kidnap him and put a doppelganger on his throne.
The closer you look at Natasha Kerensky, the less you see a Fiery Redhead who is good at what she does and much more someone who is at best a Blood Knight, and at worst a Sociopathic Soldier (though to be fair, she would have been a type 4 after her lover was killed in cold blood) — the accounts are legion of epic (and lethal) bar fights that The Black Widow and her Black Widow Company engaged in out of sheer boredom and for the fun of it. Another account concerns a member of Wolf's Dragoons admitting that he had leaked information that made its way to House Kurita, information that contributed to the Pyrrhic Victory that the Dragoons endured on Misery. Natasha cold-bloodedly killed him, just as Jamie Wolf was about to slap the man. When Wolf called her on her murder, her response was to quote words used by Nicholas Kerensky to justify the complete annihilation of an entire Clan.
Game Breaker: There are some 'Mech designs that rather tedious to fight against. A most notorious example was the Clan pulse laser/targeting computer combo that practically guaranteed hitting your target. Land-Air-mechs in hybrid mode where practically unhittable. Under Solaris rules (extreme close combat for arena battles), multiple machine guns could reduce a 'Mech into scrap metal in short order, without generating any heat points. In most gaming groups, those things where countered by house rules forbidding the use of such designs.
Before the new rules removed the bonus for them, Rotary Autocannons found themselves mated to Targeting Computers quite a bit. The ability to put up to 30 points of damage on a targeted location, or even at a hit bonus, proved to be overwhelming, and as of the Total Warfare rules, rotary and Ultra autocannons at anything beyond single fire mode can no longer make an aimed shot, nor can pulse lasers. They still get the standard fire targeting bonus, though.
Most Clantech counts, but four of their weapons stand out in particular:
Clan ER PPC's not only have the extended range of their Inner Sphere counterparts but also deal an additional five points of damage, meaning they can take an enemy mech's head off in one shot. This effectively makes them a much lighter and smaller Gauss Rifle that doesn't require ammo or explode when shot.
Clan LRM's weigh half as much and take up half the space as their Inner Sphere variants and unlike the latter also have no minimum range, making them deadly in any situation.
Clan ER Medium Lasers do an additional two points of damage over their Inner Sphere equivalents, effectively turning them into Inner Sphere Large Lasers.
Clan Large Pulse Lasers have double the range of those fielded by the Inner Sphere and, yet again, also deal more damage. Combined with the enhanced accuracy of Pulse Lasers this turns mechs that mount more than one into long-distance murder machines like the Rifleman IIC.
They Changed It, Now It Sucks: Many players reactions to the Dark Age period, especially the releated CMG. The reaction was far from universal, however, with the CMG not only bringing in new players, but many older players enjoying it and the changes to the setting. And, in all fairness, this has been a reaction to just about any change to the universe or game.
What Measure Is a Non-Badass?: Clan society is a strong in-universe example. If you're not badass enough to essentially bite and claw your way to the top and hold your place there against would-be challengers, eventually your fellow Clanners will start to consider you a liability to the Clan and start looking for your replacement. Clan society by and large puts little value on human life as such — it's how much somebody contributes to the Clan that counts, and with the warriors on top and naturally seeing themselves as the ones doing the most important work, it's no surprise that Clan civilians are usually considered second-class citizens and can be deployed, expended, or claimed as spoils of war as the warriors see fit.