I returned to Warframe a few months ago after nearly a decade away from the game. I got my brother-in-law to try it for the second time and this time around he got hooked.
It's surprising how rough multiplayer still is nearly fifteen years into the game's development. Story missions can only be played single player, but they're also the main way of unlocking new multiplayer content. Nearly every single session he'd have to drop out of co-op to do a quest or junction challenge - it got better the further into the star chart he got, but this was a constant problem up until the entire map was unlocked (which took weeks of casual playing even with me carrying him through harder content).
The story is also incomprehensible to a new player. Story bosses pop up at random places on the star chart and these assassination missions take place at different points in the timeline (never stated or explained, of course). It took ten minutes just to explain the antagonist Alad V, this game's Revolver Ocelot equivalent who has missions with him on every faction's side (including the player's).
Despite that we've been having a ton of fun. The game has a ludicrous amount of content and constantly introduces entirely new gameplay modes. We've put in probably 100-200 hours so far but, looking at the changelogs on the wiki, we're only caught up to what was released in 2019.
(Also Follie is absurdly OP. Not only can she create mission objective items, she can summon objects and gear that outclass anything a Warframe can do. This includes a Thermian RPG that does high seven figure damage, enough to one-shot even raid bosses like Eidolons. She can do this at level 1 with no mods. Meanwhile the crit-specced Vectis Prime I've Forma'd several times and filled with maxed out mods can barely hit low seven figures on red-crit headshots, and does maybe 20-40k damage on regular hits.)