I'm not saying no mecanums (or however you spell it), but these are some points made in previous threads:
-Less traction on the key
-As already said, loss in pushing power
-Heavy
-Forces you to use maximum number of motors for drivetrain (not usually an issue)
-While they may not affect your ability to go on the bridge, the ability to stay without being pushed off or falling off of the bridge tips your direction will beN tough to deal with
-You won't be driving circles around other's bots unless you have a lot of practice.
-In order to work to their fullest extent, require a lot of thorough engineering
-More complicated to code, even at the minimum level
Like I said, I'm not telling you not to use them, but when you do, be careful and engineer it through (Not meant to sound like most interesting man in the world meme, though I'm tempted.
)