I will throw my two cents into this and say I think one of the reasons why Tamers SU is able to stand on its own is because it allows crossovers. If it was strictly Sonic Underground characters and other normal people, the number of stories Tamers could do within reason would be horribly limited and dare I say would hurt the cast in the long run. Allowing the SU cast to interact with more media took what might be a relatively cringy and edgy fan series turn into a more fleshed out absurdist series. The Sonic Underground having a rival band? It's the Doodlebops of all things- and said rival band also turns out to be a bunch of creepy pedophiles. War with The Boys? It becomes an ultimate showdown of ultimate destiny between the forces of Sonic Underground and evil. Tamers was always cringy, edgy, and melodramatic; but the amount of absurdity added by seeing Sonic interact with a Doodlebop or Yugi helps shave the edge and the cringe down and oddly allows the melodrama to not be as melodramatic.
I guarantee you if you tried to make the hour-long special against The Boys just be between the SU cast and The Boys it couldn't work as an hour-long special, or so many contrivances would have to come out it'd hurt the special as a whole. That's a thought that's been in the back of my head since the start of the MLP run, it's just the Mane Six and then the 5-way gay ensemble, and I think that's a huge reason as to why the cringe and edge seems more prevalent even if its always been there, there's nothing else but these two groups to bounce shit off of- and these two groups could not be any worse to try and bounce comedy and drama off of. I couldn't put the words together until now and I hope I got the point across.