For English it's pretty solid from what I've seen, and the way it presents etymologies as coherently written readable articles is more accessible than Wiktionary, which is a lot rawer. It's nice that it goes back before written sources to reconstructions of older languages (which OED doesn't do IIRC, but Wiktionary does). On the Proto-Indo-European language front Wiktionary is slightly more luxuriant in this regard because you can search through non-English languages as well and explore etymologies a bit more freely because of this.

It seems to lacks citations as to where the info comes from, which is a bit unfortunate, but I guess it's part of its friendly vibe? But it also obscures how people know this stuff, and makes it harder to fact-check. (I'd be surprised if they didn't have the references stored somewhere that's not getting published - I imagine they're something you'd want to keep track of as you're writing the articles).

[I don't use Etymonline very much because I'm mostly looking at relating etymologies between words of different (Indo-) European languages I'm learning/know right now, rather than plumbing the origins of single English words.]