LANDR - Are they serious?

AdrianNewington
No apparent Sonic difference, and it seems that they just pushed my song through a Hard Limiter to a peak

Maybe they determined that was all you needed.
 
You didn't expect it to take Blowin' In The Wind and turn it into Dust In The Wind, didjya?

I have to admit, LANDR does a much better job than I would have expected from an automated process. Probably as good a job as your average DIY ME.
 
That said, it does take a generic approach that, as you suspect, is primarily running it through a limiter and a multi-band compressor. A good mastering engineer would be capable of far more. But LANDR's still a reasonable alternative for someone who doesn't want to invest the time to learn mastering for themselves.