Hornqvist had a tough season this past year, but Matheson had an absolutely atrocious season two years ago and, while better this past year, looked abysmal against the Isles.
If this trade does get completed, I would only feel comfortable taking on Matheson's contract and all of that risk if there are draft picks also coming back to the Penguins.
At least a first rounder next year (since I assume they wouldn't be able to get the Panthers first in the coming draft), if not more, like a second rounder this season and then the future first.