Hallmark is a large financial services company that owns the aviation underwriter known as Aerospace Insurance Managers. Aerospace has always been a specialist in low limit, high(er) risk coverage. Single ship flight schools, low time pilots in higher performance complex aircraft, commercial helicpoters, that sort of thing.
I've been out of the biz for a year, but as of that time, they didn't write many RV risks.
My opinion was that they were OK. Great, if you needed them for something that the other guys wouldn't write. Their business model is one of charging high premiums for stuff that other underwriters don't want.
They contract out claims handling, so expect slow service there.