One thing that we must keep in mind is that a roman legionnaire is not merely some guy with a gladius and pilae, just like a modern soldier isn't some guy with an assault rifle and grenades. Training, discipline, tactics, etc., are perhaps even more significant than the technological level of the equipment. While technology is, in my opinion, pretty robust (in the sense that it doesn't usually disappear, especially if it had widespread use and support), those other factors are much weaker and can degrade for any number of reasons. The training and discipline of a Roman Legion would have a lot to do with the institutions that the Roman government provided, and if those collapse even if you know what the tactics are supposed to be without the proper training they can't be effectively employed. Likewise, as rrpg points out, the Roman army was very big (compared to a medieval state) and standardized, this is because the Roman state could support such a big army. Effective tactics used by a big army might be very different than those employed by a much smaller force.
If you combine technological differences with institutional ones, then attempting to recreate roman legions may have been impractical/doomed to failure. Many of the Renaissance era commanders studied Roman tactics very closely, but none seem to have attempted to emulate them.