Even against weather teams, Charizard is outsped by choice scarf Politoed or Tyranitar. If your lead is Charizard, you will have to guess whether they are choiced or not. If you mispredict, your weather doesn't go up and you possibly lose your weather source. Your opponent knows what item you hold, so while your information is incomplete, your opponent has a full awareness of whether or not he outspeeds you. When two regular weather users collide as leads, you can tell which is faster because the slower one's weather will stay up. Since your weather doesn't come up on the switch, you lose this information while the opponent maintains perfect knowledge. You will, for this reason, most likely have to switch out when confronted with Politoed or Tyranitar in the hopes of being able to switch in later rather than risk losing your weather source turn 1. To compound the issue, if Mega Charizard does manage to become popular, incidence of scarfs on weather leads will increase to counter it. Being Fire/Flying, Charizard has the honor of being a bad choice both against Rain teams and against Sandstorm teams, so the risk of dying when trying to establish weather mid-game is still very real. Aqua Jet is particularly important to note, since the likes of Azumarill have a very good chance to OHKO in rain, even without rocks.
It's also important to mention that, since Charizard must attack to get weather up, even slower opponents with a sash or special bulk will be able to return fire. While Charizard only needs to attack once to become Mega for the whole match, that one hit it may have to endure, combined with the risk of SR damage the next time it tries to switch in, will seriously limit the number of times sun can be re-established against other weather teams. The lack of Leftovers recovery (or a sash) put it even further behind Ninetales in terms of longevity.
