* ComplacentGamingSyndrome: It can be easy to be use nothing assault rifles and sniper rifles against transgenants and Reticulans. However later on new armored transgenants and Reticulans with force-field armour start appearing and will hand you your ass if you're not careful.
** Deathbellows are probably the only Transgenant that will put the same fear in you as a Chryssalid would. For those not in the know, deathbellows are giant, hollowed out slug-things with bees in them. The bees are their "ammunition", meaning once they fire, you can't do anything about the bees themselves, which will always hit, and don't particularly care if you're wearing armor. One shot will ''half'' kill a soldier, if you're ''lucky''; otherwise, they're dead. The bees are fired at a quite frankly ludicrous rate for the damage they do, and the Deathbellows themselves are resistant to projectile, laser, and explosive weaponry (i.e ''90%'' of your usual arsenal). [[KillItWithFire Deathbellows are surprisingly flammable]], though, and their tendency to lay in wait around corners is easily their weak point as well. They are even easier to deal with in urban terrain types, because there's almost always some urban debris lying about that you can use as a shield to get close. Once you scouted out and know where the Deathbellows is, switching to your designated grenadier, getting him/her within throwing distance, and having him toss a few incendiary grenades over the cover will quickly deal with the Deathbellows. On larger wilderness maps, you can afford to stand outside the Deathbellows' firing range and wear it down with sniper rifle fire.
** In ''Aftermath'' Reticulans packing plasma guns will feel this way in the early game. Lategame, substitute Microslug Accelerators.
** Beastman Matriarchs in ''Afterlight'' can rapidly enter DemonicSpiders territory depending on how they are armed, and what you have on the field. You will particularly hate Plasma Cannon and lightning throwers if you're relying on drones.
*** You're generally going to hate Beastmen no matter what. They can almost always see you before you see them (especially during night missions), they have better range and accuracy than you do, and they absolutely love to chuck grenades at you. If you don't have Heavy Suits or Life Support Suits, you're going to die. If you ''do'' have Heavy or Life Support, you're looking at a significant healing time for your soldiers anyway.
*** Though [[FrickinLaserBeams Laser Cannons]] tend to make relatively short work of them.
** ''Aftershock'' gives us Wargot Grenadiers. Their Frag Grenades are mostly harmless, but Incendiaries will put at least one of your soldiers in the sick bay for a while.
** Also from ''Aftershock'', the [[spoiler: Starghosts]]. What makes them so dangerous is that they can use mind control to capture an ''entire country'' if you fail to defeat them when they attack a province. And you [[TheComputerIsACheatingBastard can never get the country back.]] '''Ever.'''
** High-level Psionics have [[MindRape powerful psionic abililties]] that can affect the accuracy of your troops at best, or make them start shooting at their allies at worst.
* GameBreaker: It's pretty safe to say that the late-game Ball-Lightning Launcher from ''Afterlight'' radically alters the entire game and brings down the difficulty level. Up until that point, the Beastmen are still terribly efficient opponents, and can only be defeated by carefully avoiding direct confrontations and using [[AchillesHeel lasers]] and [[SquishyWizard mind control]] properly. The Ball Lightning Launcher allows a technician/soldier with Driver Training to generate an explosive "Lightning Ball" that can be guided for any distance and any amount of time, and detonated ForMassiveDamage right in the enemies' faces. And since the launcher has an in-built thermal imaging sensor that can tell you about enemies around the lightning ball, you can pretty much win any mission with a single soldier, provided he's got enough batteries to power the damn thing.
** In Aftershock, cyborgs who max HP boosting stats and implants have absolutely ridiculous health, more than any enemy can ever hope to overcome. Add in the third-level bonuses from Gunman and Ranger training, which grants that the soldier doesn't get knocked down and can act with serious injuries, and your cyborg becomes an ImplacableMan.