Of coursa Honda has a big part of the blame for McLaren's lack of performance, more than 50% from my point of view, I think anyone that hasn't acknowledged that is either a Honda fanatic or lacks knowledge (fortunately I've never seen anybody state that). The only thing being discussed here, at least from my side, is that there's a lot of people that ignore the fact that McLaren also has a big share of the blame.
Let's not forget that McLaren has not won a world championship since 2008, so basically since the aero makeover they've always been a step behind the opposition. Before Honda, on 2013 with closely matched engines, and 2014 with the best engine of the grid, they didn't finish past 5th on constructors'. So even prior to partnering with Honda they were already lost at sea with their concepts. Also, the ones that chose to partner with an engine supplier without guarantees of performance was McLaren, and when Wazari says it was McLaren who asked for the size zero unit (with the blame on Honda for accepting it out of pride), it makes total sense. All of which just shows that McLaren has gotten where they are by themselves, and as said, they certainly need Honda much more than Honda needs them.
Then comes 2015, Prodromou moves into the team, as well as Honda's flawed power unit, and everybody just assumes that hadn't it been for the lack of power they'd be pushing for the World Championship. To me that just seems quite naive.
The chassis looks very good from the outside, and also has never shown any fundamental flaw, but a team doesn't go from 5th to a championship winning chassis over one winter. Red Bull was only so dominant because they were working on their design even prior to the rules being established, and even so didn't dominate right from the start. It took Mercedes 3 or 4 years to develop both the chassis and Power Unit that made them one of the most dominant teams in history. I stand convinced that even with Mercedes power this chassis wouldn't be a strong contender for the World Championships and it just sounds too ingenuous to think it would.