He's too small - He's one inch shorter than Cucurella. A couple of inches shorter than Marcelo, same height as Roberto Carlos.
Can't defend - Correct he's not an exceptional defender but he can only "not defend" in then same way Zinchenko and Cancelo "can't defend" i.e. he's regularly caught out positionally and occasionally in the air or one on one.
Not an upgrade on zinchenko or even angelino. How? He's more technically rangy than either of them (Zinchenko is usually - Villa aside - ineffective in the final third while Angeliño is a one trick pony - crossing) plus neither of them could defend either. Zinchenko on his day could put in an excellent defensive performance but that was after 3 or more years of being coached by Pep. We could see how much better Grimaldo could be if Pep can improve him defensively.