If you e.g. use LSPs or similar tools it usually is better to separate different projects into different Vims. Most of these tools rely on some kind of project root and may get confused if you have different ones opened in the same Vim instance.

I personally also like the separation and navigating my Tmux is very fast. To me Vim is the perfect editor but I don't want to use it as windowing or operating system replacement. Obviously I use tabs and windows in Vim but always only regarding the project I am working in. E.g. I use tabs for diffs a lot and reviewing.