It is when expectations clash that you get a problem.
If the GPs think that certain food patterns or behaviour management styles or safety guidelines are normal and don't expect to be challenged on them, yet the parents expect them to magically know what modern standards are and stick to those, especially without ever actually discussing it then you get clashes, and people get hugely annoyed with each other rather than just communicating about it and being open minded. If you can both be open minded, it makes it easier.
In addition people get excited about babies and it's such a hugely emotional thing, especially when you've had your own before, you do get stuck in your ways of how to do things and you tend to build this image up in your head of it being how it was when yours was/were little. That's jarring when someone says no, no, we're doing it this way - and if you're a sensitive kind of person you might even take it personally, like what, was the way I raised you/DH not good enough?
I'm having DC2 just 10 years after DS1, with a different partner and not only have I found it hard to accept he has different preferences over certain things, (perfectly reasonable and expected differences, I might add - but your own feelings can surprise you) I can't believe how much some advice has changed and I feel quite sceptical about some of the stuff people are advised these days. OTOH there are things I can't even remember what I did or thought best 10 years ago - that's all fine, because it's my child and I'm able to read up about what's advised now, discuss with DH, make a decision and will do what I think is best, but if I was being asked to treat my grandchild in a way that I found utterly ridiculous or even harmful, I might struggle with that, I think that's completely human. And my experience is only 10 years' difference. Parents today probably have no idea what was common advice 30 years ago and might not expect some things to even be questioned, which adds another layer of difficulty since if you don't anticipate a GP might do something in a certain way and they don't anticipate that it might be a problem, that could cause anger/disbelief as a response rather than "Oh, I should have explained to them that the advice has changed". My mother was forever trying to put DS in a seatbelt on somebody's lap. It was considered normal if you ran out of car space when I was a child. It wouldn't have occurred to her that (in an emergency) the child is safer under their own belt even if they are technically too small to be without a seat. MIL used to let him ride with the diagonal part under his arm when he was in a booster - I had no idea until he tried to do it one day in DP's car. It didn't occur to me she wouldn't use the car seat properly, it didn't occur to her that this was an unsafe way to use the car seat. To her it stopped the belt rubbing on his neck, which she felt was dangerous. (If she'd routed it correctly, it wouldn't have.)
I also think parents today have higher expectations over the control they will have over every aspect of their children's lives, this probably wasn't such an expectation in past generations, and probably that can cause some friction too - where GPs feel they "aren't allowed to do anything" but parents feel that the GPs are "trying to take over".