Well, Djokovic made 12 SFs out of the last 14 USO he played. In the early exits, he was either injured and retired while down or was disqualified. His SF+ losses were Federer (3x), Nadal (2x), Murray, Wawrinka, and Medvedev. All but 1 of those losses were to 3+ slam winners, and 5 of the 8 were to 20+ slam winners (not to mention the #1T and #2T players at the slam). His less notable losses were to PCB, Wawrinka, Hewitt, and Verdasco. The last 2 being before he was 20.
Did he underachieve considering his accomplishments elsewhere? Absolutely. Is the USO his worst slam? No. You don't have an average of over 6 wins per event in the last 15 years without it being an amazing slam. His problem at the USO wasn't the slam itself, but rather circumstance.
RG is a different beast entirely. First off, he has fewer RG titles. That's already a point against it. Even discounting every edition he loses to Nadal (which we shouldn't, but let's) he has 2 wins, 1 final, 2 SFs, 3 QFs, and a R3. Even without the Nadal losses, that's barely better than his first 9 USOs, and certainly worse than his first 11 USOs.
At the USO, he's been scarily consistent but his peak level was much lower than it could've been. High skill floor but lower skill ceiling that he consistently met. At RG, his consistency was slightly worse and he met stronger opponents. His level may have been slightly better (but even that's easily debatable).
Points in USO's favor: Actually won more slams, has a better win %, has fewer shock losses, has more finals (same win rate in finals).
Points in RG's favor: Possibly eye test, also played Nadal at RG 10 times (biggest challenge in sports).
Do RG's pros outweigh USO's? I'd say no. Certainly not enough to make up for a 1 slam difference. RG is Djokovic's worst slam until such moment as Djokovic surpasses his USO title count with RG titles.