the MOs have to ascertain whether every relevant AD has been complied with.
Indeed but checking
ADs is very different to what has been said earlier which was checking the a/c did not have any mods done off the logbooks - ever.
Checking ADs is often safety critical, and even the "pointless" ones are usually manufacturer's SBs which are generally worth doing.
I know of some Socata and Honeywell SBs which were bollox (oil pressure sensor, autopilot) because they were written by people who didn't understand the subject, but most SBs have a reason behind them. Usually the ones which would have affected in-warranty aircraft were called "non-mandatory" because only "mandatory" ones were warranty covered
IMHO, anybody who ignores ADs which are safety critical is a plonker.
Equally, anybody who gets ADs implemented and doesn't make sure the work is logged is a fool because his plane will be worth so much less when he tries to sell it. Unfortunately, most people rely on their MO to keep the logs and they rely on the MO to not disappear with them after a few years. I have had my plane just 8 years and have already had three firms (that did important work) disappear; luckily I retrieved the records long before they went down. Air Touring, CSE, one other. I think owners need to be a lot more pro-active on this front than most of them are. Even taking copies of each Annual's work pack is highly desirable.