You're hearing that from people who haven't figured out how the game works. It simply isn't true.This is REALLY disappointing to hear. I had thought that bounded accuracy was supposed to make it EASIER for the DM to deal with min/maxers.
I regularly DM for my local group, and we've moved from 3.5 to Pathfinder. They're a bright bunch of guys who tend to enjoy min/maxing, and most recently my "fix" was to play various variants of E6. The players have gotten sick of E6, though, and I was hoping that moving to 5E would keep us all happy.
Now I'm hearing that the game breaks down by level 10-12 if you have min/maxers playing it? Ugh. Not again. Why?
Honestly, there are very few minmax problems that can't be cured simply by increasing the number of monsters in the encounter. Outnumbering the party always makes things vastly more difficult for them. You probably don't even have to go outside the XP budget if you don't think that's fair.