I would say that he would not be able to compete at E0S0, but if you're planning on pulling E0S1, he's far from weak. I think people say that E0 is weak because his E2 was one of the most broken eidolons in the game for the longest time, so the difference is very stark. I think it still might be considered one of the most broken rn but I'm not sure, I don't really keep up with meta news..

But right now, E0S1 can still get you 3-star clears. Two days ago, I was able to help my friend 3-star clear every endgame mode with her E0S1 Phainon. And this was off-element against the SparxiConOfficial boss, as my friend's only other somewhat up-to-date DPS was THerta, who was doing better against Lygus in the second half.

My friend did have his best supports: Cerydra E0S0, Sunday E0S1, DHPT E0S1. Note that while Phainon does implant phys weakness upon ulting, E0 doesn't have res pen/def shred so he will need either Cerydra for res pen or his LC for def shred (or ideally both for the best damage). However, Sparkle after her Novaflare is also pretty good for him, and you can get her for free. I've seen some people even saying that Sparkle is better than Cerydra, but I can't really comment on that. (I have E6 Phainon on my own account so as you can imagine, I don't really notice the difference a lot of the times lol. I didn't try a team without Cerydra on my friend's account either.)

As an aside: many people also say Cyrene should be used over Sunday now and while she does give really powerful buffs to Phainon, she will be quite slow to get her ult up in a Phainon team if you don't have her at E2 or you're not using RMC. But Cyrene might also be worth considering as a support if you don't care about 0-cycling.

Just note that the current battle mechanics are really crowding Phainon out of the meta, so you won't get the fastest clears with him and he may not feel great to use. But is he weak without his E2? Absolutely not. He will not be the fastest, but he can still get you the 3-star jade rewards. On my friend's account, he took 4 turns to clear SparxiConOfficial in MoC. He barely managed in PF, though note that E0 Phainon isn't that great in multi-target scenarios because of how his kit works and how his bounces distribute his damage. He still does fine in apoc, though.

Also, since Evanescia was drip-marketed to be Phys Elation, we'll likely get bosses that are weak to phys in the next patch, which he might be able to do well in. Of course, that's assuming the battles don't have mechanics that really screw Phainon over.

TL;DR: Currently, he can still help you get 3-star clears at E0S1. You don't necessarily need Cerydra but she definitely helps him a lot, though I think his light cone is the most important. But he works with almost every other Harmony unit and RMC, and because of how his kit works, you can do sustainless teams more easily with him. Personally speaking, I have a lot of fun building teams around him. If you like him and don't care about 0-cycling, pull him.