Aye denons are the only army that can play in both fantasy and 40k, even more easily now thanks to the round sigmar bases.
So they're good value in that regard, but the model range is fairly limited and some of them very old (IIRC they've got a lot of metal to fine cast conversions*), and the last 40k codex from memory removed some of the randomness from them (thankfully) but also ended up reducing how effective they could be (if you rolled the right dice, tilted your head and squinted).
They're not a new user friendly army in any sense of the word, as from memory virtually every unit has special rules and lots of random rolling.
They were fun last time I played them (last edition of 40K I think), but really random and if playing seriously I could see them being very very annoying, fortunately most of my games tended to be for a laugh with friends so when I invariably rolled low on reserves (I had several games where half my demons didn't arrive until about turn 4 from memory), or did a mishap** it was largely fun.
I vaguely remember one fantasy game where the side I was on (I can't remember if it was me or my partner) rolled a mishap and it took out a demon that had tried something and because it affected the building he was in/on it also took out the good guys hero character (something like the demon upset Khorne as the mishap, resulting in Khorne doing something that affected anything within X inches of the spell caster). It's rather vague as it was about 4-5 years ago and I've only ever played Demons in Fantasy about twice.
*I've still got some of the models from the Demon army box that I gave up on painting/assembling at the time because they were really badly cast metals (the hell hounds ended up taking me ages with a dremal to get the heads onto the bodies as the bodies were squashed one way and the heads another at the joints).
**I really suck at reserve and deepstrike rolls, to the point where whenever I roll for them it's almost always going to go wrong, which is a shame given that I've got two armies the really strongly rely on deepstriking and reserves (although the new reserve rules and latest demon codex does make it slightly better).