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.
Sanctum 2 patches have this effect. Coffee Stain Studios tries their best to avoid the Complacent Gaming Syndrome, but reactions varies whether they succeed or just annoy people who have created a solid strategy to solo the hardest stages of the game.
It's hard find someone who doesn't use the Drone Launcher. However a patch has severely toned down the Drone Launcher making it much less powerful than it used to be, it's still one of the most used weapons, though.
The Focus and ACP towers were almost mandatory when playing in maps with Feats of Strength. Not having at least one of these usually meant other players immediately leaving the game. However patches have toned down the power of these towers or/and increased their cost.
Walker Pups, Runners, Snorkers and Brood Mothers. Small and fast nuisances that generally come in such large numbers your towers alone will not be able to handle them as one or two are bound to slip through towards the core, consequently forcing you to leave the frontline to deal with these pests.
Armored Heavies and Soakers. Both are slow and have a LARGE (and shiny) weak point, but both also have large pools of HP and will make your towers focus on them instead of other threats which they can actually kill - unless you set them to specifically target the weakest enemies around.
Rhinos can outright one-hit kill anyone but Haigen with their charge attack if one doesn't pay attention. The biggest problem when dealing with them is the fact they always come in waves with other enemies that might distract you.
Spore Pods. Seriously. A single Spore Pod is harmless, slow moving and easy to destroy, but unlike the other fliers they spawn in ridiculously large numbers. You can be doing perfectly fine, with a defence network capable of repelling any ground force with ease, when all of a sudden you get a double-wave of Spore Pods and they simply overwhelm your defences by sheer numbers. A single round with double Spore Pod spawns basically FORCES you to invest heavily in anti-air.
Spitflies and Screamers in Sanctum 2, who actively attack you (and the Core, most importantly) with projectile attacks, often en masse. Some maps become extremely difficult if you don't find a way to counteract them quickly, the fact they tend to attack the core earlier doesn't help either.
Screamers also have an "upgraded" variant: the Screamer Matriarch which not only has much more health but also some armor to reduce most of damage to boot. Some levels throws you a lot of these things in a single wave basically ruining any defense network not crafted to deal especially with this situation.
Mutators. They don't attack you nor the core. Instead they power up other lumes and can drastically increase their health hundreds of time, to make things worse they have a very erratic walk pattern and will almost always walk into a group of very aggressive lumes. Oh, on certain maps they come en masse.
The Walker Patriarch boss. He will prioritize bases and towers unless you walk into him and shot him a few times. He can royally screw your strategy by crushing your carefully planned and built labyrinth and creating a huge gap in your defenses.
Super Heavy is no slouch either. Although slow and very easy to bait and kite (be careful as the attack can destroy bases and towers) he's also extremely tough with a very small weak point in comparison to normal Heavies. By the time you're finished dealing with him you'll be either rushing to defend your core (possibly under attack) or praying to kill him before he reaches it. Unless you have the Drone Launcher or an upgraded Drone Tower ready expect to have a huge headache dealing with this thing.
The Hovering Queen is a Hoverer on steroids. She is armored in the front and can only be harmed by being attacked from the back, however the major problem she can cause is with her attack: it's a shockwave that not only will knock you very far but will also disable your towers for a brief moment, enough for other critters and enemies to slip through your defenses and attack the core.
Train Station is a straightforward level with absolutely no way to build a maze. It's proven to be a level where the player has to know how to crowd control huge waves of enemies with little help of towers as enemies tend to speed through the defenses.
Abandoned Lab has a very erratic layout and two cores to protect, when playing solo it is almost impossible to take care of an entire row and the player will find himself running back an forth to defend the cores at all times.
They Changed It, Now It Sucks: A lot of fans of the original game were pretty unhappy at the tweaks and alterations made to the gameplay in the sequel on release, most notably the expanded focus on the FPS elements, the fact that you could only have ten towers maximum on the field at a time and resources (split into tower bases and tower resources proper) had to be retrieved from the core manually, which made timed waves a lot more difficult to work with. Fortunately Coffee Stain Studios actively listened to the litany of complaints and quickly patched in fixes which seem to have made the fan base happy.
Constant changes and fixes by the developers to tweak powerful strategies makes a lot of players unhappy about it, especially those meant for "5 Feats of Strength Solo" variants.