I can already tell which of my friends are likely to end up as someone's nightmare MIL. They have a weirdly co-dependent relationship with their adult sons and still treat them as if they were little boys, not grown men. That doesn't bode well for when a permanent partner and DGCs comes along.
One of my friends has a son in his mid 20s who works in a graduate position in London. He phones her every single day on his way home from work, on his walk from the tube station to his flat. That's 'their' time for a daily chat. If I suggested to any of my adult children that we had a daily chat, let alone a regular time for one, they'd get a restraining order out on me, I think. WTF do they even talk about, daily? I find that bizarre.
I've had perfectly good relationships with all of my children's partners so far, no grandchildren yet which makes it easier, but we don't live in one another's pockets and we are very careful not to offer unsolicited advice or opinions on things that don't really concern us. And we never, ever say anything that might be considered critical of their partners, even though at times we have to just 'sit on' something that's giving us concern.
I have friends who never seem to like their sons' girlfriends. The concern is always that she is too controlling and emotionally manipulative, that he's being dragged into something too serious, too soon, no longer has any time by himself or time with his friends and all the decisions for him or about him now seem to be made by her, or are too heavily influenced by her.
We've had a bit of that with one of our DC, whose life choices and career and future seems to be based around prioritising their partner's needs. Their partner has decided that they need to move to a specific city for a career opportunity, so our DC trots off after them, even though it's a bit of a tumbleweed place for their own job opportunities. This is just about to happen for the second time.
But in the end have to be very careful how we broach that concern, without it looking like we are encouraging them to split up, which we are not. So we keep our counsel. In the end they are adults and can make their own decisions. If it turns out be the wrong ones then all we can do is be there for them when it goes wrong and try not to say I told you so.