I do neck ferrules instead of plates from time to time when asked for. You put them in just like string ferrules.
I like 1/2" or 5/8" diameter ones for looks and most you find are quite large. I believe I got my last few at Bitter Root guitars to get the smaller ones. They are a bit of a pain to me when plates in comparison are virtually no work. The look is clean though.
Just be aware they take shorter screws and using the standard ones can bite you.
As above, with the caveat that it also depends on the neck pocket and thickness of the body/platform for bolting. This would rarely be an issue except in an unusual design, as in the case of some body carves deliberately intended to remove lots of material from that very location. Though the plate is overkill most of the time, it is inherently a stiffer and better design. Done properly, countersunk bolts work fine; done improperly, they can exercise a splitting force on the body. I have several guitars done both ways, and no problems, but I actually prefer the plate. I'm not building any more, but after reading this thread I think I would countersink a plate!