Hello and welcome to the kitz forum!
Has anyone successfully used DSLstats with the XMG3927?
Yes. It works as expected. No real issues to note after almost a couple of years of stats retrieval. I have been using DSLstats 6.5 for a while.
What functionality can I expect; HLog, QLN, Bit-Loading?
All the usual functionality possible with a Broadcom chipset should be available. On VDSL you should have full DSLstats functionality, unlike on G.fast where QLN, HLog don't work properly, bitloading doesn't get graphed at all, and, as expected, VDSL-specific pbParams requires VDSL. Didn't I say no real issues? Well, for me those things are trivial.
I don't know if you're planning on buying an ISP-branded device or not, but it may be worth steering clear of one of those as Zyxel states they don't support them directly. But that's entirely up to you.
