The more one reads about the Exarchs and Seers of the Throne, the more they seem like the good guys who wanted the world less supernatural so the common man could live without being in fear or under the control of the tyrannical Mages of Atlantis. Until one remembers that the Seers' tenets include "Regulate the Abyss" and "Divide Humanity," it's not clear if the Exarchs even really exist, and every attempt to create a new Atlantis has met with bizarre and/or catastrophic failure even without the involvement of the Seers. Then, looking further, you realize their motives weren't so much to eliminate tyrannical Mages as to concentrate those Mages into their ranks.
The Keys to the Supernal Tarot sourcebook has alternate interpretations for the characters, Legacies and items inside, depending on how the Storyteller wants to use them.
Aaron Murphy is the leader of the Huntsmen cabal from Banishers. In a sourcebook where most of the adversaries are portrayed as somewhat sympathetic, he stands out for being a rapist and torturer who uses the Banisher ethos as an excuse to indulge his sick desires. He is presented as a total sociopath, willing to deceive and betray anyone for his ends. Even his backstory of parental abuse and fear of female Thyrsus fails to make him any more sympathetic.
Joe "Blood of the Lamb" Beal is a Serial Killer Scelestus from Boston Unveiled and Left-Hand Path. An absolutely insane Thrysus, he started out bad, with his Awakening involving seeing Jesus Christ as an apex predator, and got worse, with him switching devotion to the Abyss because the "natural cycle" most Thrysus respect didn't give him enough power. He's a master of psychological and physical torture, and what's worse is that he doesn't even have a motive beyond sadism—to him, his prey are just toys he can inflict various Life-related tortures on until they beg for the end.
If you become an archmaster, you are now encouraged to be this trope. Turn your soul into a Genius Loci! Take a vacation in the Abyss! Visit the Supernal Realms to change history just by being there!
Game-Breaker: For the beginner mage, Extended Casting's geometric progression of effects can be horribly abused. By the time you get to master-level magic you can break the game into tiny little pieces without even trying too hard.
Even with that in mind, all along the progression of power, the Mind Arcanum is just plain better.
Time and Fate Arcanum together can make a lot of non-combat encounters trivial, and at higher levels combat itself becomes a joke. Being both the ruling Arcanum of the Acanthus Path just piles on more cheese potential.
Every spell in the Time Arcanum above three-dots might as well be named "Derail Plot" in the hands of a sufficiently intelligent and motivated player.
Second Edition pretty much turns any mage into this with just two dots in Time, thanks to the Temporal Sympathy attainment. What this basically means is, if you have your target at hand, you can with little difficulty affect them at any point in their past with a spell. This includes effectively preventing the person in question from even being born in the first place without the spellcaster themselves suffering any negative effects from the change. Combine this with a few Fate failsafes that effectively screw over whoever tries to prevent said spell, and you practically have a character who with relatively little effort can derail a campaign on a massive scale.
Probably the worst game-breaker is if the players simply ignore magic altogether beyond 'paying dues' and exploit their way into a mundane trade. A matter mage "discovering" some rare earth deposits can crash half the world economy while becoming wealthy enough to buy off the Seers themselves in weeks, with only starter dots in matter. Likewise, any time the players can frame the conflict in terms of other splats, they'll tend to win.
Technically every Arcanum is a Game-Breaker at the higher levels, especially when as many as four or more are used together, but that's the point really: Why should a Mage play by reality's rules? This gets balanced out by Paradox, and the fact that all your enemies can do the same things you can. They may not have the exact same powers, but that just makes them more unpredictable. And let's not get started on the beasts which come out of the Abyss. Another balancing factor is the Pax Arcanum. Simply put, all of the really high-level Mages, whether or not they agree with each other, have in place a kind of mutually assured destruction pact that is rather aggressively moderated. While there is a great deal of play in what a Mage can and can't do, going too crazy with high-level magic is a really fantastic way to get a group of Archmages to blow up your face.
Growing the Beard: The 1E corebook was considered mediocre at best, providing a rather poorly-explained setting that felt considerably weaker than Ascension. The supplements coming up after this provided more information about the lore and setting, and fleshed out the various factions, eventually allowing the game to grow its own fandom. The 2E corebook was overall considered excellent and a huge improvement regarding the rules and mechanics.
Night Horrors: the Unbidden parodies the hell out of them with False Awakenings. The sufferer believes they have awakened on a path no one has ever heard of before. They gain power at an amazing rate (read: from Gnosis 1 to Gnosis 9 in a week)— and ultimately destroy themselves and a sizable portion of the surrounding landscape trying to reach the Supernal. Poor deluded bastards.
To a lesser extent this applies to Low Wisdom, High Gnosis Mages, in their own heads... To everyone else it's just A God Am I with real god-like powers.
Spatial or Temporal Sympathy. All a Mage with two dots in Space and/or Time needs is an object or a person you've been in contact with (the longer you've been in contact with it, the better, but even something like a pen or a glass you touched for a few seconds can suffice if the Mage is skilled enough or has sufficiant Yantras), and suddenly he can use the former to cast spells on you even if you are miles away from him and in a different building (including to scry you) and the latter to cast spells on your past or future self. Seers Prelates from the Panopticon Ministry are even worse, as they don't even need such objects; they have a permanent sympathetic connection with basically everyone, allowing them to spy and cast on whoever they want.
They Changed It, Now It Sucks!: When the corebook came out, it was generally seen as mediocre and inferiors to Mage: The Ascension due to the loss of emphasis on post-modernist, subjective reality in favor of a vague "Atlantis was real" myth and the switch from the popular and sympathetic Technocracy— generally considered the highlight of the oWoD's villains— to the (at the time) selfish banality of the Seers of the Throne. Thanksfully, the various supplements, and later the 2E corebook, fixed most of these problems.