Not good enough.
Physicality is not the issue. Nadal was a beast at 18 already.
I don't understand your logic here. Because Nadal was a beast means your average 18 year old is likely to be as well? Hasn't he long been held as an exceptionally physical specimen?
Between the slow state of the courts and the highly physical nature of the game, I'd say physicality is a glaring issue.
Those factors lend themselves to other issues as well though, too, like a greater need for a big weapon. In the absence of that, a rising player better be pretty damn versatile since weaknesses are made harder to hide as points, on average, seem more difficult to close out assertively.
For those reasons, I believe Raonic was able to make a name for himself on tour sooner than some of his seemingly more talented contemporaries.