I suspect that most high stakes hold'em players use preflop solutions such as Monker. Whatever range an opponent has, we can hold fixed EV and learn easier postflop GTO strategy by imitating Sims.
However, preflop GTO is not completely solved. The main reason is that the game tree of hold'em is too huge, but from the perspective of solving Nash equilibrium, I think the bigger problem is multi-way possibility. Theoretically, we cannot solve any Nash equilibrium in multi-way situation.
For example, when you 3-bet from SB against BTN, there still remains a player(BB) behind you. So if you want to do some GTO stuff here, you have to assume that BB will fold to 3-bet 100% of the time. Then heads up with BTN and SB, now you can solve the Nash equilibrium. But, of course, this premise is unrealistic. Because nobody folds AA, KK, AK, or maybe KQs from BB against SB's 3-bet.
So my questions are;
Do Sims really have reasonable performance even in multi-way situations?
How are Sims made actually?
What do you guys think about this?