If the ubercharge build on arrows proves too powerful then they should make the 1st arrow give 10% and all further arrows give no more than 3-5% for like 4s or something, that way you still get a lot of ubercharge for hitting that big arrow but can't build way too quickly on lasts.
Phlog isn't a good unlock when compared to the old degreaser but I think people are underestimating its use when you play around it with a team. Like you can whip and uber a phlog pyro with the crits and it fucks everything. (obvs not in 6es but whip and phlog are banned anyway lol)
In any case the core mechanics of pyro are pretty dumb and the whole class needs a pretty big rework to be a viable class that doesn't rely on weird gimmicks.
I think the same could also be said for engi and heavy though, it would nice to see the classes do something more than just "build area denial machine" or "be area denial machine" without being completely broken.