the massive difference is that seraphim were added by original developers of the game, they are canon and part of the game no matter how anyone feels about it, they were also introduced like less than a year after original game afaik, they were basically part of it from start so idk how anyone could have a problem accepting them, unlike nomads who are completely fan made in every way more than a decade later and unofficial and having nothing to do with original developers
I also found seraphim confusing at first and I still can't tell the difference between many of their units and buildings as easily as for other factions, but they are part of the game that always existed for me, and for anyone else coming to FAF, literally the intro of FA shows seraphim and the entire story is about seraphim, everyone who played the game anywhere knows about seraphim because they are everywhere in it from start (even original supcom mentions them repeatedly, some critical stuff in story is literally seraphim), same can't be said about nomads or other potential fan factions, many probably don't even know nomads mod exists, especially those just coming to FAF