That's a question that's tough to answer, but i'll give it a try.

Most enemy variants are straight up more dangerous than the vanilla units. Often just a bit, but it adds up. The AI also does its part to make the game harder when compared to vanilla.

I think that Better ADVENT should probably drop you down one difficulty (so from Legend to Commander)

I agree with DerBK. Also depends on other mods you plan on using. Can you give us more details? I'm running ironman commander right now and it's brutal

I don't use any other gameplay mods. I have ~80+ hours in LW so that prepared me. Vet/I was easy after I learned the new units, C/I took a few tries and now my second run on L/I. I cheat every once in a while by force closing the program.

"C/I took a few tries" if you mean without ABA then make that several tries with ABA.
I generally agree with DerBK. Commander/ironman is really hard. Much harder than vanilla legendary. I'd suggest you go with Bronzeman mod tho' considering ABA is still in development and it'd be a bad idea to have something happen to your game in ironman mode.

The AI changes are pretty brutal, especially with the most recent patches, especially in the early game when you lack the equipment and troops to deal with enemies flanking and overwatching constantly. Things get easier once you have access to higher level troops and things like gas grendaes and flashbangs to help break up stalemates.

Yea sorry, I meant vanilla. I haven't tried using ABA at all yet. I'll try it on C and see how it goes. Are there any significant releases coming up ~soon that I should wait for before giving it a try?

You can start now. We don't do updates that require a new save. New changes will be applied every time you update the mod

any other gameplay mods in particular you recommend w/ aba?

I recommand "Additional Icons",

http://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=646244015

It displays the unit name which is really handy with all those different variants running around.

so I tried on commander, got ~4-6 hours into the campaign and found it to be a bit too easy. After maybe ~6 false starts on legend, I got a decent start and it seems to be going well enough. My main issue on commander was that the 'right' way to play xcom is always to kill the aliens before they have a chance to act, which is still pretty doable on commander due to lower hp so I didn't see a big difference from vanilla.

This is also coming from someone who did ~80 hours of LW, and beat xcom 2 on vet/commander/legend before trying the mod.

I think I'm in a similar position to the OP in that I want a relatively vanilla gameplay experience with it just being more challenging. I personally played through one normal legendary run before running L/I's until I won twice.

I put on a couple of the map packs and I'm thinking about putting ABA on on L/I to see how it goes. I'd love to know how your missions are going so far in comparison to my own run on the new maps I'm going to be trying this mod with soon.

I had a lot more false starts with ABA on as the AI makes far fewer obvious mistakes to 'gift' you a fight. Now that I'm actually 'going', things are pretty stable and it's a nice upgrade from L/I vanilla. Overall, it's a good change and is fresh enough to warrant another playthrough while being a bit more difficult as well

Found out that this Mod is too OP for me. Seriously, in my second mission of the game (after I had completed the main objective) I had a officier and 2 stun lancers drop in. How is that remotely fair?

The Chryssalids are the only critter that my mod lets appear earlier than in an unmodded XCOM2. If you saw Stunlancers and a Captain, you would've been able to see them in the unmodded game as well.

I prefer playing on veteran with increased pod sizes, you'll end up with less "kill them instantly or lose a soldier" gameplay and more firefights with both sides getting shots off