It's entirely on the height IMO, which I'm not going to deny is a big obstacle in that. Mostly height, a little bit skating. Not that it's bad but needs to be well above average to negate the height/reach.
Consider this:
Rossi played on a stacked Ottawa team in the Eastern Conference of the OHL. He played with a likely top 10 pick, 52 goal scorer in Jack Quinn, and Quinnw wasn't even top 3 point per game producers on the team. Excluding Rossi, there were still two 1.5+ ppg producers in Ottawa, and Jack Quinn at 1.44. Cole Perfetti's best support player was Nick Suzuki at 1.52 (which would have been 4th best in Ottawa), and that was over only 23 games in Saginaw. The best player in Saginaw for the full season was at 1.29. He played in the Western Conference, which had a higher competition level than the Eastern Conference by a healthy margin.
Take out the 4 worst teams in the OHL this year: Niagara, North Bay, Kingston, and Sarnia (3 EC, 1 WC) from their production this year.
Rossi drops from 56 games @ 2.143ppg down to 41 games @ 1.805. A point per game drop of 0.338.
Perfetti drops from 61 games @ 1.820ppg down to 50 games @ 1.700. A point per game drop of 0.120.
We know Rossi is at an advanced physical and mental level than a majority of juniors level players. He tore up the bad talent, with the support of pretty elite company as far as the OHL is concerned. Perfetti was nipping at his heels against the better talent in the league, with significantly less supporting cast.
This is why the offensive upside of Rossi I feel ends up over hyped, inflated, or exaggerated. I think he was a driving force of his team to be certain, but I think his elite production is more of a product of his situation, more so than it is a reflection of how high end his ability is.
And this is actually a flip from where I was earlier. I took the stance that points were points, and there wasn't enough behind the numbers to be worth looking into. That Rossi was still performing against better teams. But the extent at which his drops, and the fact that his team had damn near a first top line's worth of players better than anyone not named Perfetti on Saginaw's roster...it's really hard to ignore.