clarry: But now that decades of effort has gone into emulation, community patches, etcetra., do you still find games that are impossible to run? Which games?
None that I can think of.
There are so many third party tools and everything that anything can be run. Obviously there are some glitches and some features may not be available as originally intended. But on the other hand, in many cases there are some new extra features, such as widescreen hacks for 4:3 games and so on.
A game that simply wouldn't run at all, that I haven't personally seen yet.
Darvond: Space Quest IV: This game is infamously broken in many places. Even running it on period accurate hardware will still not be perfect because oops, your bus speed is slightly different, here come the time cops.
Space Quest IV is a very annoying game as is, but I don't think the problems are related to compatibility.
In fact, there are so many ways to run that game on modern systems.
DOSbox, ScummVM, fan patches to fix things ("New Rising Sun" and others), etc.
Unless you want absolute perfection, it's actually an example of an old game that is easy to run on modern systems, rather than an example of something that is impossible to run.