Along with the presence of magic, one of the big things that sets Rifts apart from the rest of the far-future, post-apocalyptic RPG pack is its inclusion of mecha. The game itself never uses the term "mecha", preferring the more unwieldy nomenclature of "robot vehicles" and "power armor". It seems that Palladium has reserved the word "mecha" for the Robotech RPG (and its related games, Robotech II: The Sentinels and Macross II, both long out of print), which is a shame, because it's a lot easier to write than having to say "robot vehicles and power armor" over and over again.
Anyway, where was I? Oh yeah: giant robots. Rifts has them. Lots of them, in fact. I'd say that at least half of the vast amount of books published for the game have a back cover blurb that proudly announces that yes, this one has new mecha in it. It's fairly unusual for game set in a post-apocalyptic world to emphasize mecha as strongly as Rifts does. Since Rifts is a setting where almost any enemy you encounter is able to deal incredible amounts of damage, armored vehicles (including the robot kind) are a basic necessity for most human characters, if they expect to survive.
Don't get me wrong: I love mecha, and I love that Rifts includes them even when it doesn't always make a ton of logical sense to have them stomping around all over the place. My problem is with the way the game handles them; more specifically, they generally have a ton of M.D.C. (Mega-Damage Capacity). They slow combat down tremendously, because the only way to take them down is to whittle away that M.D.C., which usually takes many, many attacks, most of which are actively and individually defended against.
In other words, Rifts mecha have too many "hit points". There are optional rules that cause malfunctions once 60% of a particular piece of the mecha's M.D.C. has been lost (did I mention that they have M.D.C. by location?), which is a nice thought, but also means that you have to crunch numbers during play to figure out, say, what 60% of this particular mecha's right arm M.D.C. is. It's fiddly, clunky, and overall quite unlike the fast-paced mechanized action that I think the game needs. The worst part is that I don't have any good ideas on how to fix it, because I don't want the mecha to blow up too easily. Just more easily.