I believe nematodes can be a very effective way of control slugs
(previous posts largely agree). However...

After a few applications do the nematodes reproduce? I.e. are there
diminishing returns after, e.g., three applications?

Do nematodes get totally wiped out after winter (British)?

I would be interested to hear if someone had tried a course of
nematodes succesfully one year, and not bothered the next - but noticed
a continued decrease of slugs.