RockingAroundTheXMasTree
Yes, it's tricky, and the idea of actually shielding a ex who is being a shit, and a shit parent, really rankles.
I think though in most cases, it's probably best in the long term. Not to get all flowery or anything, but to try and keep certain facts to yourself.
I'm basing this on a few things - firstly just my own experience with divorsed parents, even though my mom tried to be very neutral, she didn't always manage, which I totally understand, but it made an already difficult situation worse for us, and it even impacted our relationship with the extended family on that side though they were helpful and reliable.
It's also been my observation that where kids are successfully sheltered from these things, they are generally grateful when they get older, they realise what went on and really appreciate that it wasn't part of their life or affecting their relationship with the other parent when they were young. And similarly, when a lot of that stuff is put on the kids, it sometimes really ends up backfiring and affects the relation with the reliable parent negatively.
None of that is really fair, but it's just not the kind of thing that can be managed in terms of fair.