We had some issues with the gun in the early days but those were early production guns and our logistics system wasn't up to speed yet but all that said, the guns had a remarkable up time during ops in Afghanistan.
In Ukraine I'd say that they are probably firing at longer ranges (and thus higher, more punishing charges) then we do here in training. On top of being ridden hard, I expect they tend to be put away wet - i.e. running crew and wpn tech maintenance is probably minimal. It's high tech gear with a lot more finicky parts than the C3.
I doubt some of the posts above re numbers except the Oryx ones which I tend to give credibility to. The fact that almost half of the Ukrainian destroyed towed guns are M777s is interesting as the Ukrainians had far more older towed Soviet stock which I presume they employed heavily even as ammo starts to become problematic. It'll be interesting to see some of the AARs coming out of this.