Simple understandable operation, they can be stripped in 20 minutes with a couple of spanners & a set of hex keys,
Lay the parts out & it's obvious how they fit to make it all work, & how basic it actually is.

The fine tuning & fettling is another matter altogether , but the base level spring - pushes hammer - hits valve, could hardly be more basic,
I've just built one up from parts, not many guns out there that you can do that

Finally is the something that cannot be quantified,
they just feel "right"

Oh & helps that they're blindingly accurate