I have beaten the original DOS version a few times, I believe at least twice without cheating. I’m not sure if I ever completed an OpenXcom run. It does become a bit of a slog towards the end. There’s two things I’ve found off-putting:
– Those endless crash missions, they do become boring
– Psychic attacks out of nowhere
The first point – I’ve started thinking that maybe it is not obligatory to clear out every crash site. Once you have enough monthly points anyway (shooting down UFOs counts) and don’t need to discover tech etc, or don’t need elerium, etc etc, mayyybe it’s ok to not do every crash site.
The second point – maybe the psionic training stuff you mention helps to mitigate that, but by that point I’m usually pretty fatigued and find constant mind control attacks unfun.
Still the best game ever made.