Look at Sylveon's eye shape, not color. The rest have thin eyes, Sylveon's are big and anime-like.
They were saying colour so I was using colour
But hey, let's tackle that whole shape thing instead then...
Cool. It's the only evolution to have the same shape of eyes as the actual eevee itself sooooo... still pretty much part of the family if you ask me.
I find a lot of the sylveon complaints a bit odd to be honest. I'm not a huge fan of a few of the eevee designs but my reasoning is usually just "I don't really like it" as that's the legit reason, so it's odd to me to see people coming up with bizarre stuff that's often just some some totally made up rules. It'd be like if i didn't accept any eevee-lution that doesn't have something representing the mane so Umbreon, Espeon and Glaceon "wouldn't look like they're part of the family" based off that totally arbitrary outline.
I mean heck, we're talking about a series where a fish turns into an octopus and people are trying to argue that something doesn't 'belong' because they think it's eyes are a lil bigger or something

It all just reads to me as people trying to make up 'objective' reasons for not liking something rather than simply admitting they subjectively don't like it.
The whole "simplicity' point I see trotted out sometimes is equally baffling. I mean, Vaporean looks like they tried to stick on as many fin frills as they could: all down the back, round the neck, both ears and one I always felt was a bit extraneous (personal taste here, it's obviously not extraneous to other peeps) right on its head. I'm surprised they didn't add frills under each leg too. Do I think it's a bad design? No. But anyone who points to it as simple has probably never tried to draw it accurately from memory: it's not exactly kirby.
I'm not even sure why simple is even wheeled out as a good thing either, I mean, as long as it's not complex enough to be an eyesore it doesn't really matter a lot to me personally.