It's not at all underrated
Saying Federer's 5×3 is underrated is like saying Djokovic's holding 3 majors simultaneously is underrated.
5×3 is part of the greater whole which is 18 - which is constantly (and rightfully praised to the skies)
Furthermore, given being an 18 time Slam champion and Career Grand Slammer, it'd be easier to 5×3 than not.
If he wasn't 5×3, I'd say he'd underperformed at at least one of the tournaments.
----
And yes, clay is by far the most unique surface. Not in terms of something as factual as how many tournaments are played on it but in terms of the skill sets required to shine on it.
Put mathematically, there's less correlation between success on clay and success on hard or grass than there is between grass and hard.
You expect a proficient grass courter to do well on hard courts (and vice versa to a slightly lesser extent).
Clay court prowess, by contrast, doesn't necessarily accurately predict success off of clay