From a gameplay point of view - Mordent is one of my favourite domains. If you want a weekend-in-hell, then there's some great stuff here, especially with the way the Weathermay family is described here. Jules is a stunning character, the elderly but still sharp quasi-ruler of the place, he's devoted his life to governing, but can see ghosts after a brush with death a few years back and is starting to suspect a lot about Godefroy and the House. But he's got a case of noblesse oblige, and his descendents are all more interested in monster hunting rather than duty to Mordent, and one of them is probably a werewolf, and his son-in-law is being blackmailed by Godefroy. There's a really great story to be told here, the last cathartic crusade of Lord Jules as the career politician takes up the sword again, and the next generation of Weathermays having all their secrets come out and deciding what their destiny is, and a climatic expedition to the House on Gryphon Hill to face Godefroy.
As a setting for a longer campaign, it works very well too. It's not as darklord-centric as many domains so even low-level PCs have stuff to do, there's plot hooks all over the place, and the nature of the place means that PCs will likely be meeting the same NPCs a lot, which gives you time and scope to build and flesh them out. The sort of Mordent campaign I'd run would be very local - again, drawing from Austen a lot, fleshing out all the local families, the PCs neighbours and servants etc, and having social interaction, etiquette, reputation, the marriage market etc be major story aspects. If Mordent seems to small for this, well, there's lots of room to expand it. As well as the sidetrips to pocket domains or Cornish dwarf-ghosts mentioned above, you could 100% put primeval stone circles, and secret druidic sects dead or living, and mound-tombs of the iron-age ancient dead, bog mummies, spectral black hounds, and dark fey here, without compromising the feel of the place at all. And Mordenheim is just across the border too.
What the place does lack is an urban area. In Regency literature there tends to be the far-off big city (London) that acts as a source of scandal, excitement, desire, and occasionally wickedness that contrasts the whole rustic gossipy neigbourliness of the small town where the actual books takes place. I'll never understand why nobody writing the Ravenloft line during its various reconfigurations and conjunctions etc saw fit to plonk Paridon down at one end of Mordent to fill that role, it's a perfect fit, but it never happened, and Dementlieu is a long way away and culturally very different. I'd be sorely tempted to have a fairly reliable mistway running from Mordent to Paridon if I ever ran the place, but whatever.