If I could get a penny for every time I compared these metrics… 
Of course neither GA nor we do own the true value here - while Google surely is undercounting (mostly due to add-ons but also cookie consent, and lateley apparently also the Safari browser), we are slightly overcounting (e.g. we have a harder time to count non-logged in visits on mobile / desktop as a single visitor). I might be wrong with this but I feel that the general trend is that GA is being blocked by more and more users these days.
In general, I do see the delta here to be around 30% - 40% when it comes to visitors. Page views somehow can differ, for reasons not clear to me (yet). The way that you handle your cookie consent surely plays a bigger role here. I suspect that the difference is even bigger (up to 60%) if your cookie modal by default does not include GA tracking. Next to that, probably the target audience plays a role as well (e.g. young and techy B2B customer vs. slightly older B2C end user).
I hope you understand that for us it is also a bit hard to give general information about the difference, as this seems to vary a lot depending on the context of each community. We will keep an eye on this in our benchmarking as well.