Like Xianxia, /hgg/ mod is also built on the bones of Revamp/UEE that came before these. /hgg/ does a lot more cleanup of UEE's bad code and grammar though, and fixing lore inconsistencies. Style-wise, Xianxia is very chinese-light-novel, I hear the genre itself is called Xian, with cultivation and absurd over-the-top super mega beam of the divine jade god attack stuff. So many unsexable duplicate golems to fight so you can battle thousands of enemies at once and put those billion damage poitns to use. Xianxia pushes harder into ignoring consistency to pump up off-the-wall stuff.
/hgg/ keeps things grounded and closer to CoC1's general vibe, adding shield bashes, weapon abilities, tons of spells, so on, but just not off-the-wall cultivation things. A lot of effort goes into checking over the writing and code before updating, so it runs significantly better with fewer bugs, and has fixes and polishing to interface itself. Content-wise, /hgg/ has toggles for watersports, gore, other ages, furry(disabling furry makes them monster demihumans and kemonomimis, except for hellhounds that are just fully feral with no anthro details). There are I think 9 new bosses and 11 new enemies so far, plus a number of new unique characters like a moth girl and your daughter, a couple undead girls, so on and so forth.
Player customization expanded a lot, so making a new save from scratch is better than transfering a vanilla save, so you can see all the new game stuff.
Transformations are expanded and the code itself is overhauled so TF abilities are more balanced and useful. Just added bat TFs and weeping willow TFs in the last update, but there are mermaid TFs, dryad TFs, etc.
Lots of perks.
Corruption content is a lot to tackle and a complicated subject. You can kill people, if that's any indication. Completely sexually break some. One of your daughters can be rendered socially catatonic after she accepts she's just your sex object and shouldn't resist. You do have options if you want to be bad, and in different ways.
Next update, one of the scenes I wrote lets a corrupt player date-rape a gay man if they're female and that was some fun.
Corruption-only characters get more content too, like Ceraph having many things added including new sex scenes and utility.
However, all that said, I've put more attention into corruption-neutral stuff with just flavors of text difference for your corruption level, and expanding options for pure players. If you're pure, but want to enter the Bizarre Bazaar, just suck the guard's cock, that's proof enough you're not too prissy for the place. Or you can beat him up, but he's tough.
Hope this was clear enough info.