The DLC/Seasonal content isn't quite just cosmetics; it's mostly moreso sidegrades.
The biggest downside from my POV is that PvP can't really be great b/c of how limited and cheesable the combat engine (and overall design) is. And that'd be fine since I've never played PvP on this and would never want to, but Funcom wants this to be a gritty PvP game, so huge amounts of the PvE gameplay would get jerked around by their PvP whims. Their big expansion that added a new map was a poorly-thought-out and poorly-implemented grindfest with RNG in place of content, and from what I heard didn't even do what it was supposed to do for PvP b/c they fundementally misunderstood the risk vs. reward that they'd offered. That said, I'm led to understand it got better when they moved to the Seasons/Battlepass model they now have, but I walked away about 6mo before that, and couldn't be lured back by it. It's a frustrating game that's poorly designed on a SE level, but it's just good enough to make you want it to be good even though it isn't. It can suck you in, but the endgame lacks content even with their current more frequent release model - endgame PvE is just building bases big enough to lag the server.
I say all this as someone who started playing in early access and logged an appalling 2000h+ in the game. It's definitely possible to really enjoy it, but there's a lot to (rightly) dislike as well.