I was lucky. We did our ground school with EuroUSC but did our flight test and OM approval with ICARUS (Aerial Motion Pictures), who were absolutely superb. But due to a combination of other commitments and the total lack of aerial work, the new price hikes have confirmed my decision not to renew next year. I feel a lot of the regulations and requirements for OM's are way OTT for the tiny drones we have now like the Mavic. Basic safety sense and adherence to law is needed, but a lot of the stuff just administrative fluff. These aren't real aircraft, just glorified radio controlled models. Some of them in the case of the Spark, barely bigger than a human hand.
There seems to be an obsession with ridiculously long and complicated OM's. No wonder the CAA are taking ages half the time! The trouble is that a lot of people seem to be writing their OM's in order to satisfy the CAA, rather than as a manual for their own company. What gets forgotten is that you are supposed to know your OM insude and out. I don't care who you are, but memorising the multiple 100's of page tomes that some people have isn't practical. How can you possibly remember all the overly complicated stuff that often goes in when you are in the middle of an emergency? OM's as we know them are grossly impractical for the purposes they are intended for, and it is generally the fault of the NQE's and a few people who were probably the sorts at school to write 100's of pages of non-writing for their English homework thinking that length equals quality.
The priority of OM's should be based on the K.I.S.S principle. Although I'd make a fair bet that apart from mandatory updating for firmware etc, most people barely ever touch or read their OM's (be absolutely honest now).