I don't use fully premade characters, more like a list of bullet points of things I think would combine to make a fun character, but I know how to make them work. The key is to actually adapt the character to the setting and plot at hand. All the problems that come with premade characters stem from people wanting to keep them exactly as envisioned in the premade form. Sometimes certain concepts don't make sense in certain settings or plots, and characters that don't actually have any firm ties to the setting have problems creating ties to the plot. If you want to bring a premade character into a roleplay then you have to be willing to change them a lot to fit.
For example, one time I saw a person who wanted to bring their premade super powerful genius mage who was the star of his class in magic school into a dark and gritty fantasy world where the player characters were on a quest to slay an evil necromancer. Their character sheet was a clear copy and paste from elsewhere: it was laid out differently than the CS skeleton the GM provided, it didn't give all the info that the GM wanted (no equipment or non-combat skills lists, no explanation of how or why they became part of the quest), and nothing in their character sheet referenced the setting at all. The furthermore, the GM clearly stated in the setting info that magic was rare because it was only ever taught in a direct mentor/student manner, so over the years the number of people who knew magic grew only very slowly as a single mage could train perhaps a handful of students in their lifetime, plus there was as whole thing about age = power level. I don't know if this guy even read the setting info though, because here he was submitting a character who was clearly stated to be attending a magic academy and was supposed to be one of the strongest mages ever despite only being 17. You can imagine how the GM was displeased. The player tried to force the GM to change the setting to accommodate his character rather than being willing to change it at all, so he ended up being rejected from the roleplay amidst a pretty heated fight.
The thing that anyone using premade characters needs to understand is that the setting should shape your character, not the other way around. Unless the GM specifically invites players to change things then you've got to abide by whatever rules and laws of the world they've set up. And if you're not willing to truly adapt your character to a given setting, then you have to recognize when your character doesn't actually fit and pass up that roleplay. Trying to force the setting to change to fit your character is only going to cause annoyance for you and the GM and probably the other players. As ironic as it might sound, adaptability is key to playing with premade characters, so adapt to the situation or don't join the roleplay. It's that simple.
