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.
The Sniper Rush (investing heavily in expensive snipers early on in a modern age game and using them en masse to instantly kill enemy units in hordes) act like this.
Towers are also incredibly powerful and almost unstoppable except for otherwise fragile siege units and massing them can provide an impressive defense.
There is no friendly fire in this game. There are, however, nuclear weapons. Say hello to saving your armed forces from direct assault by nuking them, leaving your enemies destroyed and your forces strangely unscathed.
Aircraft carriers have a limit on how many aircraft they can produce... but it only applies when the aircraft are onboard. Send them all out, build some more, and it can still carry the old ones, evidently having infinite Hammer Space hulls.
Fortresses (not available in the campaign for obvious reasons) allow you to store units inside, which removes them from the population cap and later returned, going over the cap.
Narm: The voice acting in the first game is wince-inducing.
That One Level: The second scenario in the Russian Campaign in the first game. Very few people were able to beat that without using a trick to create a duplicate of the scenario and activating the ability to cheat in it.
The second scenario in the Asian campaign is even worse, as you have to complete a long list of highly failable objectives on a tight resource budget while assailed by unlimited enemy reinforcements. At one point your citizens rebel and to make them stop you have to delete part of your military. Saving a region from economic collapse is hard.
Empire Earth 2 attempted to introduce a plethora of content and game mechanics (including a gimmicky minicam). While the end result remained at best well-done, it made gameplay more confusing and complicated.
Empire Earth 3 meanwhile tried to streamline the gameplay while expanding on the historical scope...only to wind up a significantly dumbed-down mess (such as removing a lot of what worked in previous entries) that pretty much killed the series.