It is also based on his high strikeout rate and relatively low batting averages at most of his levels. 29% K rate last year. That was a warning sign that Cameron Rupp's batspeed was not good enough as well.
The power is nice, but as I said, everybody hit for power in AAA last year. HRs increased by almost 60% in AAA last year. If Grullon only hit 15 HRs in AAA would we consider that special? His AAA performance was decent, but by no means great (and his throwing was the best part of his fielding - the rest needs work).
You have also not answered my questions about his tools. He is a heavy, slow guy. Do you consider him the rare exception that gets better at the major league level in spite of his tools and why? He could be more than a backup catcher, but it sure does not look like that from both his "production" and his scouting report/tools.