So, I don’t have fused relics, but I have a ton of fused legendary stuff… meaning, if I’m running a legendary build, it’s likely all fused.
I say it’s not as fun - not because I can’t win a that level (that really would suck). I say it’s not as fun for several other reasons…
The matches take much longer. Much much longer. I like 'em fast. LOL
The queues are longer. Yawn.
The players are saltier LOL I mean… seriusly.
The builds are more “meta” or whatever you want to call it. They have to be to compete at that level. I can bring art builds into lower rarity matches & still compete. You can’t do that with legendaries/relics. So, the visual variety & “beauty” disappears. You go from playing something that is visually akin to MadMax to something that is visually akin to cardboard boxes shooting at each other.
It’s more complicated to build. This is a pro and a con for me. When I have time to really tinker with the game, I’ll build something where all the synergies among parts really works well. It takes a lot more attention and time to build a good legendary build. On the other hand, I can slap together a 7k build and compete fairly quickly. I have 14 blueprint slots. My “heavies” have been pretty much the same for months. I’ve adjusted them, tweaked them, etc, but the basic build has stayed the same. Why? Because it’s a lot of work to rebuild one of those.
My lighter builds get rotated almost every other week… meaning completely starting from scratch.
Speaking to your point, though, you’re right as well as wrong.

Wrong because of what I just said.
Right, because what YOU said is the experience of most people when they first stick their toes in the waters where the heavies lurk. You walk in there with your first set of legendary guns & get absolutely trounced. Why? Because you barely have an understanding of how the mechanics of the game work & you go against dudes who have fused builds with absolutely perfect part synergies… wheels buffing cabs buffing hardware buffing weapons… and fused. Fused beyond belief.