GMs shouldn't care about PC dice pools really. The GM can always just toss 10 more dice if needed, they're the GM.
I personally have never GM'd that way, and never will. It comes down to three points for me.
1. If a player hyperspecializes in one aspect of a build, it generally means they want to be top of class at it. If I, as a GM, just wantonly up the threat level to match the optimization then I am essentially saying that what you want our of your build doesn't matter.
2. There are
always other ways to challenge builds. Outside of a high Karma defense Mystic Adept in SR5 (no longer works in SR6 for a myriad of reasons), every build has combat holes. You just can't plug them all, and this is particularly true in SR6.
3. It just doesn't make sense. If you look at the stats of the PR rating 8-10 NPCs, these guys are like the fucking cream of the crop 1%ers of the universe. Only truly unique and big named characters swing harder. If you end up facing these guys every single Mission, just because Steve min-maxed, it personally obliterates the integrity of the game setting for me.
I expect that others will view this very differently, and that is ok, and leads me to:
Well, that's OK in a one-shot, but I never run those. My current campaign just passed two years with a stable group. I'd feel downright shitty asking someone to play with one arm behind their back for the long haul.
Missions is just a very different sort of game, is all.
Bingo.
In a home game setting, it is important to make sure that all the people playing together have similar play styles and similar desires for the experience they want out of their shared game. Discussing this ahead of time, and having the insight to recognize when someone will just not be a good fit and politely expressing as much is the key to heading off almost all compatibility issues.
In Missions, or any other living campaign, you need to be tolerant for a few hours sometimes. If you can't do that, you shouldn't participate. Getting upset because a player is being a problem is one thing. Getting upset because of how someone built a character is completely unacceptable.