I'm the developer of Cydia, an alternative to the App Store for jailbroken iOS devices, a software ecosystem which was filled with both modifications to existing clients and complete re-implementations of clients, and this is definitely the #1 mistake that new developers would make (much to my annoyance, as it just seems so OBVIOUS of a thing to not do, but people really are idiots): you need to be
extremely careful with trademarks and logos--only ever talking about a client "for" some other service as opposed to using that service's name as a modifier on your app (so, "YouTube Vanced" is definitely
never OK and a
dumb decision when you could say "Vanced for YouTube" or even be extremely safe and just stick with "Vanced", leaving any mention of "YouTube" to your descriptions), etc.--lest you get wrecked. And yet, somehow, you see it happen time and time again as people just can't help themselves when it comes to using other peoples' trademarks for their clients.
And yes: it is an incredibly simple thing to avoid, and yet it is also a deceptively difficult thing to fix and cleanly walk away from later when the other party actively wants you to not exist as you are often now stuck in a really shitty position with respect to the hole you have already dug yourself into and the legal fees you are potentially going to lose just trying to defend yourself going forward. Hell: someone I don't like from the "extended" Cydia ecosystem (if nothing else: they are the kind of person who thinks they are above trademarks) once even just hinted on Twitter at building a product with the name "Snapchat" in it (which I obviously would never have sold as I was really careful about this sort of stuff) and their lawyers started coming after me with a cease and desist for something I wasn't even going to do, costing me thousands of dollars to deal with (as their lawyer knew to directly contact my lawyer, making it harder for me to avoid any costs).
And yes: it is an incredibly simple thing to avoid, and yet it is also a deceptively difficult thing to fix and cleanly walk away from later when the other party actively wants you to not exist as you are often now stuck in a really shitty position with respect to the hole you have already dug yourself into and the legal fees you are potentially going to lose just trying to defend yourself going forward. Hell: someone I don't like from the "extended" Cydia ecosystem (if nothing else: they are the kind of person who thinks they are above trademarks) once even just hinted on Twitter at building a product with the name "Snapchat" in it (which I obviously would never have sold as I was really careful about this sort of stuff) and their lawyers started coming after me with a cease and desist for something I wasn't even going to do, costing me thousands of dollars to deal with (as their lawyer knew to directly contact my lawyer, making it harder for me to avoid any costs).