While your comparisons may be similar on Pyatt vs Matthias, Pyatt was expected to have top 6 potential and hopefully find a fit with the Sedins. Matthias obviously isn't expected to have that role, but if he was brought in, it'd be as a third liner so we'd expect that level of play.
Mathias is like Pyatt. Big but below average at everything but not terrible in any area. Pyatt believe it or not actually hits a lot more too.
Lets take Pyatt's second season with the Canucks. He wasn't playing with the twins but scored 16 goals, was 4th on the team in hits, 2nd amongst forwards in blocked shots and he killed penalties, +9. He was 26 years old that season.
Mathias scored 10 last season, was a -2, killed no penalties, was 14th on the team in hits, 2nd in blocked shots.
Considering how unpopular Pyatt was I foresee similar fate for Mathias. Neither player is bad, useless etc., it is just big men tend to be very unpopular in Vancouver because they are expected to be Todd Bertuzzi.
Pyatt, Bernier, and all the players brought in with at least the thought to be tried with the Sedins at some point had too much expected of them and when they didn't pan out, they fell out of favour. We do have players that only really fit in the top 6 and will have similar reactions from the fans if they don't do well enough, but for players that can play those depth roles, we should expect them to start there and anything else is gravy.