Hmmm hard to say where charizard stands tier listwise especially considering that no has a definitive tier list for PM in general. His weaknesses are that he big (and therefore easy to hit), has trouble dealing with lots of pressure due to poor defensive options to get opponents off him once they get in his face, hes a heavyweight but also kinda floaty so that in combination with being a large target makes him combo food once he starts being comboed.
Id say roy wins against charizard generally though im biased as a roy main so i might be wrong, id imagine samus to be pretty rough since charizard doesnt have any solid, straightforward aggressive options and generally relies on his heavy punish game where samus can just camp it out and zone charizard, starving charizard of interactions but again im not certain on that.
Charizard seems to have trouble in dealing with matches that keep him from being able to play a passive aggressive style (i.e. the other character has a better passive aggressive style and more options to force charizard to be the one to approach)
Im unsure what your looking for when you ask what are bad situations for him because id say that bad situations for him are, for the most part, just generally bad situations and arent character specific to charizard.
