1. Some feats are OP, Sharpshooter and Great Weapon Fighter would be on this list, I would probably add healer feat these days.
Since 3e on there's always been feats that can become problematic. Actually since 1e UA with it's weapon specialization, double weapon specialization, point-blank shots, & Cavaliers.... And they differ by the group too.
But! All that's required to prevent those problems is that the DM pay attention to the characters being played /options being used & design adventures (or modify existing ones since WoTC claims they don't assume Feats/Magic Items when they write) so that proper challenges will still be had.
There IS a learning curve to this on the DM's end.
2. Feats in general break the game (in the PCs favour). Big offenders here are warcaster, resilient: con, and healer as they tens to negate the concentration mechanic and pacing of the gamer.
On this you're wrong. Feats change the game. If the game breaks because of them it's the DMs fault. See above.
Personally on the concentration mechanic? So what. My preferred edition is 1e where spells just have a set duration, so (as the DM) I don't
care if there's some feat that all but negates the concentration mechanic. I'm used to spells simply continuing. Now if they drop? Bonus! As a player? I'll only consider taking those feats only if those are what best helps me express a particular character. They haven't been yet.
The healer feat? Really? I'm the DM, I can always dish out some more damage.
3. Dex is stupidly OP unless feats are used (assuming you are melee, there is still sharpshooter).
The only real change to Dex here in 5e is adding it to your ranged (& some melee) damage.
Otherwise, same as forever: +s to AC, +s to initiative, +s to hit with range.
You, I , & others will just have to agree to disagree. Maybe you found this to be problematic in the games you ran/played in, but this has not been my experience to date.
5. The encounter guidelines did not work that well past level 10 or so (or 7 perhaps).
I won't disagree with this. But then I don't think WoTC has ever produced a decent set of encounter building guidelines. So I'm not shocked they missed it again in 5e. They won't manage it come 6e either.
6. High level 5E was stupidly easy even ignoring the encounter guidelines with combats X5 deadly.
This is mostly a DM style problem. Also not a new problem as various DMs I had were complaining about it way back in the mid-80s. Mostly it's the result of the DM not taking into account what the characters at their tables can actually do & writing/modding things to challenge that.
So here we are 3 years later, was I right, wrong somewhere in the middle?
Somewhere in the middle.