I think it's gotten significantly better as time goes on. Going back to the SNES/Genesis days, ports (either direction) would often be an entirely different code base, so it'd be the same game only in spirit.

I also don't recall very many PSX/DC/N64 --> PC ports. MGS and FF7 would have been some of the few that I remember from those days.

It's definitely better today than it was 10 years ago.