Personally, I don’t know why JustSim gets a rap as a mid-tier developer (I’ve heard that thrown around before). I guess it depends on what you call top-tier, how many tiers you have, etc. I bought the JustSim version and it looks outstanding as do ALL of the JustSim airports I own. Are they FlightBeam or FSDreamTeam? Not quite, but they’re in the top tier for me and I don’t hesitate to buy their work now if it’s a project I want.
Maybe they weren’t as good in other sims?? I dunno, but in MSFS they’ve done beautiful work.
Short answer, if you get it you will not be disappointed in the least IMHO. As for the Diemos comment, I’ve never bought one of theirs but I agree…from what I have looked into of theirs, they’re maybe slightly better than default.