Charizard is solid. He combos and tech chases brutally and has the 5th fastest run speed (under Sonic, Falcon, Fox, and Metaknight). He also can make it really hard to land safely on platforms cause up smash is super good at covering that. For edgegaurding his on stage edgegaurd is pretty good cause f-tilt and d-tilt, and his offstage edgegaurd is great cause he can go deep and still be fine on the way back.
He has terrible horizontal air speed and mobility which is his main weakness. Being fat isn't the greatest either. Also, while nair is one of the best moves in the game (the move is insane), it fails to fit the typical gtfo attribute most nairs have, and as such he can have trouble when pressured in the air.
He has sort of a hard time dealing with projectiles because of his air speed (it means the gap between him and his opponent has to be smaller before you can approach through projectiles with an aerial).
That's about all I can think of for disadvantages.