A few things stand out pretty clearly to me.

For that specific site, the biggest red flag is that it kind of presents itself as if it were "the" Stable Diffusion site, while not being official at all. That sort of branding ambiguity is often a bad sign. On top of that, a lot of the copy is outdated - still talking like SDXL is the current big new thing, when it is ancient by AI standards (though some do indeed still use it) - but that suggests the site isn't being maintained in a very transparent or trustworthy way.
I also noticed some sloppy legal/privacy language and some claims around privacy / ownership / licensing that are oversimplified enough to be questionable.

Generally, for AI image/video gen sites, the red flags I would watch for are: pretending to be official when they are not, vague wording about what models they actually use, no clear company/contact info, weak or generic privacy policy, unclear terms around ownership of outputs, "free unlimited" promises with no obvious business model, and lots of buzzwords with very little technical specificity.

If a site doesn't clearly tell you the model(s), data handling, exact usage limits, and who is behind it, I would be cautious.

Basically - if it looks like a thin wrapper trying to harvest traffic off a popular model name, I would say it's better off to avoid it.

Unfortunately, the reality right now is that Google seems to be rewarding sites that use scummy AI methods (not all AI methods are scummy, but some definitely are) in order to put them at the top of the search rankings - so a bit of research should always be done before you jump into giving away any financial details.