I don't think any of them are difficult to work with. The K31is probably the most complex of the three but it's not bad. Mauser bolts are no problem. I never really heard much about headspace issues with the Enfields either except from guys that don't know much about them which seems to be common on some of the other groups. They do have different bolt heads and they can get switched which is what some say the problem is with some importers that just slap things together. The vast majority are probably just fine.