The main challenge with the world building, particular in sci fi vs fantasy, is when it is so exciting for the writer, that it bores the reader. They just don't keep up with made up words and lots of new creatures descriptions or artificial countries etc. It's not believability per se, it's just too challenging to process, particularly on the net when people read haphazardly and multiple unfinished works at the same time.
The moment the prologue gives me the 'there will be a test later' vibe, I drop the book.