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.
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.
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.