I think enemy variety and the various timings of when they show up might need some looking at. I'm not sure what LW2's underlying mechanics of its decisions for what to throw at you when, but here are my experiences from 1.5 Veteran campaigns:
Overall note of mission types completed - Two full liberation chains, two other chains through lib3 to unlock network towers, somewhere between 6-8 troop columns, 3-4 supply raids, and the usual plethora of guerilla/resistance lead missions.
ADVENT- Tons, and tons, and tons of advent. Like seriously, I love that you've expanded on advent variety, but holy hell, that's all there is, it seems. Semi-related so I'll throw it up here, are MECs. This overall has seemed fine. Their first appearance seemed appropriate, and how often they show up has also seemed appropriate.
Drones - Love that these got added back, I missed them from LW1. Seems solid/appropriate.
Sectoid - In my current campaign, Sectoids make an appearance in Gatecrasher, then I encountered maybe 4 total until May, then ZERO (or maybe 1 single on a supply raid, I forget) all the way through early/mid November, which is where I currently am. I already wasn't super happy with the change in the visuals/model for the sectoid (I miss those cute little grey dudes), but now I'm just like... where are my sectoids? They were the bread and butter of XCOM.
Vipers - These show up in insane numbers comparatively to Sectoids. Perhaps that's a side-effect of increasing their variety, but I'd like to see more parity between the two early alien types.
Mutons - This is spot-on, I think. Vanilla XCOM2 was sadly lacking in Muton presence (I missed the football teams from LW1), so seeing more mutons in LW2 is encouraging. Related so I'll throw it here - Berserkers seem fine.
Andromedons - I'm mid November and have yet to encounter one. This frustrates immensely. To have key techs locked behind this autopsy (and THEN another corpse retrieval for each thing you want to build?) seems absurd, with their relative scarcity, combined with the lotto of which missions they might actually show up on (corpse retrieval or no). I'd like to see some tuning here, perhaps a Mk1/Mk2 split, with Mk1 starting to show up earlier, and with appropriate presence -- probably something similar to how often you see higher tier MECs, zerkers, higher tier Mutons, etc.
Gatekeeper - Never seen one in my 1.5 campaigns. Not even sure what to say, this "Long" war isn't long enough if a reasonable campaign-finish timetable puts you well before Andros and Gatekeepers show up.
TLDR: I think overall, there needs to be a re-shuffling of weights for enemy types. Too much advent, too few of certain aliens, with special focus on Andromedons (and possibly GK's).