Are all fetishes problematic?

After seeing the thread on foot fetishes it made me think how can one distinguish a harmless from a harmful fetish? Obviously if you're sticking gerbils up your ass that's pretty fucked but if they want to suck on toes that doesn't seem too bad.
Where is the line?
I personally don't have a problem with any fetish as long as its between consenting adults.

Obviously this could trigger some people so Viewer discretion advised etc etc

 

I mean if they asked nicely and had clean feet. Not the first thing that comes to minds though ahaha.