Thinking about some of the "controversial" married ins to European royal families in the last generation:
Letizia - a divorced, not religious TV journalist married into one of the most devout royal families. There are still a lot of people who don't like her and rumours that she and her MIL don't get on but she's never attacked the people of Spain, the media and the family publicly because of it and does her job.
Mette Marit - Had a drug past and a young kid. Now has a chronic illness which means she doesn't do many duties. She has complained about the media intrusion of her son Marius (who then tried to use his name to be a promoter) and other things but she hasn't attacked the family as unsupportive.
Durek the Shaman - He and Martha-Louise have complained about racism in the media and in Norway but have emphasised how supportive her family have been. Worth noting that the media mostly has a go because he's a conman who peddles dangerous cures not because he's black. Ari Behn also never attacked her family despite the divorce and him being very controversial. He said they were great.
Maxima - Terrible family who were banned from the wedding for her father being part of a Junta. She has her ups and downs with the public and media (Greece trip, "too busy with the UN to care about the Dutch people") but doesn't attack the Dutch media or whine about things. Is still in contact with her family privately.
Sofia Hellqvist - does not attack the Swedish media for calling her a porn star when she started dating CP. Does everything in her power to prove she's shaken off her past. A section of the population still doesn't like her but she's won a lot of people over. She and CP were gracious when Carl Gustav took away their kids HRH and seems to get on well with his parents.
Daniel Westerling "The Personal Trainer" has proved himself and never criticised anyone for the way he was treated for years even though it's claimed the King still doesn't like him much.
Chris and Madeleine's popularity took a dive when she moved to Miami and still wanted her perks but they actually live a quiet, private family life and she still does occasional work for the family. Chris knew he didn't want to be a royal so refused a title. They don't start suing when the media say nasty things about them.
So The BRF can be forgiven for thinking that even though they didn't think Meghan was suitable for the role, that it would work out in the end if they gave her public and private support.
No one else has had to deal with a couple attacking the people of the country, the media and the family all at once, as well as trying to be celebrities in a different country and needing to alternatively trash them and suck up to them depending on the day. They want to leave royal life because it's "toxic" and they need to keep repeating that to get attention but then they also want all the perks like titles, being there for the big events, and want to be seen as adored by the people they trash and scream racism and abuse when they AREN'T invited back. They also want to be fake royal and do their own engagements. No one could really have foreseen all of it, even if many predicted they'd move to LA before long.
Even Sofia someone who likes publicity, is not married to the heir and with an embarrassing past has stuck with it.