If you're old enough to remember Darius Kasparitus, that's a good comp for his upside and type. For more recent players maybe a smaller Cernak or a spicier Carlo type. Those comps are more about impact than style because he's pretty unique stylistically.
He's trended like a 4-5 at best in Montreal, but there's flashes and earlier in his career signs of maybe a #3 upside. I think this trade will be helpful for him to reach his ceiling, as he'll get a really long leash with Dobson which is a great opportunity.
Good skater, 5'11 210 brick shithouse kinda frame, delivers big open ice hits and is very stout on the wall and in front of the net. He makes an alright first pass, but really hasn't shown much offensive upside (there were flashes under St. Louis, I'd be curious to see if he can nurture this next to Dobson). He sometimes tries to make the big hit too often, but he's a good skater in general so he can usually recover. Interesting player, I think he got overrated quite a bit in Montreal, but there's a solid chance that the Islanders just got themselves 5 years of team control on a cheap #3-4 LHD they can put next to Dobson and then have two dominant pairs. I think he's gonna learn a lot from Pelech about restraint and playing smarter too.