i don't like having to race to the server to avoid playing a class i explicitly don't want to play. i also don't like potentially being captain, especially if people alias or i just actually don't know them.
fatkids also are an issue. an extreme example would be if there are 11 high intermediate and invite players in an advanced pug with rainoflight. whoever is forced to have him on your team will lose. this is just an example
adding to classes and captain were always the best advantages (in my eyes) over inhouses that irc, pugme, and pugchamp did well.
i mostly want to add on classes and not be RNG'd to captain. matchmaking queue systems don't work properly when the playerbase is tiny. i would like to say i'm not a baby, and these qualities didn't stop me from pugging, but it's definitely an influencing factor to the "should i play tf2" question