The biggest problem is that web2py openly encourages learning practices which do not transfer over into general Python programming and, in fact, are the literal opposite of how actual Python programmers work. Things like magic imports seem great at first, of course, but in the long run A) get you lost quickly if you ever have to write real Python and B) have the same problem any bit of hidden magic does: sooner or later something's going to go wrong and you're going to have one hell of a time figuring out which bit of implicit "helpful" code is causing it.