I was holding a great chokepoint on hills (a single hex) surrounded only by high mountains with several MG units but, despite this, I was swarmed and encircled quickly and easily by enemy raiders militia, cutting my supply line. I think this is a problem.
Umm... what's the full scenario? Did you fully ID the units? Was it a thick mountain chain? Etc.
Inf can and will move through mountains. When you see inf do that and it will take a little time, react. Shoot them. Their readiness will be rubbish.
If you weren't in the middle of the Himalayas, then some minors get fast units like buggies and tanks, specifically raiders and slavers (and nomads but they get cav, like actual mounts to ride). They may have gone round the mountains or they may have eaten the AP and readiness penalties to drive those vehicles into the mountains. Etc.
There are a ton of factors at play. Mountains are tricky to move through but they are not literally impassable.
In short, to answer your original question, ZOC is very weak in this game. ZOC will not protect you. You have to actively defend your flanks and keep reserves to prevent breakthroughs. You can use terrain to secure your flanks but know that all terrain is passable to determined infantry, if nothing else, so terrain is a deterrent to flanking not a barrier (except water/lava/any-other-freestanding-fluids-that-maps-can-generate-that-I-am-unaware-of which are truly impassable). Personally I like ZOC as it is. It's there and it means exploiting a breakthrough etc. is not completely trivial but it is not some overwhelming factor to the point you have to play dodgems to have any hope of opening a gap wide enough to exploit. It also means that securing flanks and keeping reserves are rewarded because the enemy could breakthrough and exploit in a single turn. You've got to be ready for that if the enemy has breakthrough potential.
EDIT: just checked in-game. Wheeled units such as buggies should not be able to enter high mountains. You either misremembered the situation or have encountered a bug. Again checked, my fresh buggies can enter low mountains but not high mountains. Bikers also count as wheeled and cannot enter high mountains.
Could be there's a bug with bikers. More likely you misread the terrain and were nowhere near as protected as you thought. If you've got a save, check the terrain and if it is high mountains and bikers really entered it, report it as a bug for further investigation.
Actually that mountain range was really like the Himalayas, and I was strongly overseeing the only road access hills hex available but, in a single turn, enemy militias managed to outflank (on high mountain hexes!) my MG troops and encircle them, cutting down supplies. For a game that emphasizes realism and simulation it's not, imho, the best outcome.
Edit: just checked, no terrain misread but enemy units were all mixed bikers, buggies and rifle militias. So maybe is really a bug.
< Message edited by YogSogoth -- 6/11/2020 11:35:59 PM >