What they are weaker against is knights.
Edit: I'd still mix pikes into a cannon stack, such that cuirassers don't completely screw you the instant they show. Also, combat II knights (very easy for AI or anyone to reach) will fight any practically promoted cannon at 50% odds. If they have combat III you actually get fairly miserable odds w/o the pike cover.
The high drill helps keep you fresh after attacking/killing units as well as defending cities vs superior #'s.