Brennan Lee Mulligan does it by fudging, if I understand things correctly. He improvises very much, too. His group seems to really enjoy it but it's a very different playstyle from the more "gamist" styles where players put care into their character's stats and such. If you sit down to make a character for a Brennan table, never ever think "Ooh, I wanna take lunging attack, that seems like an effective maneuver". That'd be meaningless. Instead, only think about what you would think would be fun or cool to do, like "oh, Rakish Audacity seems like it'd be a fun play pattern".

Colville's approach is similar to Brennan's.

Mercer's official statement, on the other hand, is that he only fudges for new players. He is generous with revivify which solves a lot of problems, too. That said, he rolls secretly and there's no behind-the-screen cam that can tell whether he pads HP or whatever. But as far as how he officially describes his game, he plays to find out. He preps situations, the players play them out, anything can happen. In that kind of game, your choices for your character matter a lot more. It's more akin to my own jam than the Dimension 20 stuff because of that.

Me, who doesn't fudge, I just have made peace with how sometimes the monsters steamroll the PCs and sometimes it's the other way around. That was my "solution". + when making custom monsters, or converting monsters from other games, I've started using the Forge of Foes approach of lower AC and higher HP. Often makes the fights feel a li'l more interactive.

I'm also scared of falling into "MPE"—"my precious encounter" syndrome, where you put so much care into creating an encounter that you end up railroading the characters into it because you don't have the heart to see it go to waste.