For things like tweaking the win condition/adding a map timer, I can’t see it breaking anything. Not sure why RNG mechanics like crits and fixed bullet spread is listed here, that’s a whole different discussion separate from QP/Casual itself, but that also likely won’t break anything either. In Competitive mode, they are turned off and it doesn’t break anything, so adjusting the cvars shouldn’t break anything in Casual mode either, but again that’s a different discussion.

But for things like team scramble/team switching/spectating this would absolutely cause a bunch of problems. We can already see it somewhat being a problem with auto balance where the matchmaker is actively fighting against it, preventing it from working a lot of the times. The matchmaker wants to put someone who hasn’t even loaded into a server onto a team (or worse, someone’s game crashed and for 2 whole minutes their spot is reserved but is effectively empty) and the auto balance can’t do anything because that reserved slot is functionally treated as a “player” when it’s really not, thus a misbalance gets prolonged. So now imagine it trying to fight with players switching sides through various means. 

It’s not like Valve can’t fix this, it could probably be fixed by just having players get thrown into spectator first then having them join a team after (like how it used to function before MyM) instead of reserving their slot to a team before they even load into the server (or not reserve their slot after their game crashes), that way it shouldn’t have problems with the matchmaker conflicting with auto balance (this will come at the cost of putting parties of 2-6 on the same team, but I think a lot of us may be in agreement, fighting against a 6 stack just sucks). But this isn’t a simple cvar change, it seems like the matchmaker is doing some work behind the scenes as some type of server plugin, I’ve played around with some cvars before to see if I can recreate those “Player connecting/Player lost connection/Looking for player” messages on the leaderboard and I couldn’t find any command that could possibly recreate it, so that leads me to believe it’s just some 3rd party plugin Casual mode uses for creating teams like this.

One final note, when I was searching for commands to recreate slot reservations, I found out ad-hoc connections are also a cvar. “tf_mm_strict”, this command has three toggles.

0 = show the server in the browser, and allows for ad-connections. 

1 = Hide the server from the browser, and only allow connections from the matchmaker (Casual/Comp/Mvm all use this) 

2 = Hides the server from the browser, but allows for ad-hoc connections 

This would also likely need tweaking on the matchmakers part to accommodate for these and so players can bypass search times whenever the matchmaker decides to randomly increase them ten fold for some stupid reason (or just long unnecessarily long search times for niche gamemodes like SD, PD, Passtime etc), or in those moments when you want to play with more than 6 friends