Thundergod
Hall of Fame
Tbh, though there might not be any grass court specialists in the vein of a Thiem, ARV, etc, I would argue that the ones who have grass as their best surface(or close to it) are tougher than clay specialists.The thing is that there are way more clay court specialists than grass court specialists. It's not Fed's fault, and it shouldn't diminish his greatness on the surface either.
Outside of Thiem, how many clay specialists ever consistently challenge the top players? (Not even counting Nadal) I think people like Kyrgios, Cilic, Raonic, and Querrey are tougher than all the specialists after Thiem.