As much as I liked M initially when she appeared on the scene as I thought she was a breath of fresh air (I didn't know anything about her before as I'd never watched Suits), the fact that she has SO many broken relationships behind her, should have been a huge red flag to H. I imagine the RF probably tried to point this out, but it was never going to dissuade him from continuing.
You could argue that the crazies on her dads side are just that, however she seems to have been given some huge opportunities by them (I understand she lived with her dad from 11 to attend prestigious private school, uncle got her high profile internship), then something drastically went wrong meaning they don't speak.
Then her mum appears to have been her only blood relative at her wedding, surely they could have squeezed in a cousin or two and an aunt and uncle?
Her ex husband has also hinted that she ditched him very abruptly, and never looked back.
As we have seen from his antics in the press over the years since his mum died, H is headstrong and extremely impulsive. Any suggestion that M would not be a good match, would likely propel him closer to her.
With every #SussexRoyal statement, it appears H (as a member of the RF) is becoming less and less useful to M's status. Unless there is another pregnancy within the next year (and I do think there will be, soon), in his fragile mental health, I think he is absolutely terrified of losing Archie and M, and will do anything to make sure that doesn't happen. It makes for an unbalanced relationship in which one person (M) holds all the cards.