Without trying to make excuses for Honda, for they truly did underestimate the size and complexity of the challenge, and thus have f*^@#d up badly, they really did set themselves an almost impossible task.
The other 3 manufacturers had a 3-5 year lead in R&D on Honda, and of the other 3 manufacturers, only Mercedes got it right on the first go [and not the least reason for that was a certain Mr. Brawn who knocked together a dream team of engineers solely devoted to producing the best PU].
Then in it's first couple of years of competing they were ham-strung by the Token regulations, which restricted their development potential. During which time they discovered they had gone down the wrong development path with regards to energy recovery and the turbo charger requirements for optimum efficiency. Indeed the whole energy recovery side was a failure. The nail in the coffin for their original design was the CCI combustion technology that Mercedes and Mahle had developed, which has meant not only did they have to develop a whole new architecture for the engine, but they found that the head design for this sort of technology is a black art, to which they were not privy.
This year they are having to put out the bush fires of unreliability that are a result of the very immature technology they bring to the track, on a race by race development schedule! This is all compounded by the need to develop new lines of code to optimise the power harvesting and delivery of the PU, with each new development they bring to the track!! I know how stressful a normal CLUB race meeting is, I can't begin to fathom how stressful it must be for the whole team [McLaren side included] to keep on top of developments as they unfold on an F1 race weekend!
So, embarrassed as I am about Honda's abject failure in public, I see it as a sort of achievement in how far they've actually come. Let's remember, when Renault showed up for the first preseason test in 2014 they did little more running than Honda achieved in 2015. And if you consider development and R&D time in the equation, today's Honda PU is at the stage that everyone else was at the start of the 2014 season.
I still shake my head on remembering all the lyrical "a jewel of an engine" talk back in 2014! Talk about getting it wrong Mr Dennis. So in a sense, Honda's re-entry into F1 was doomed to failure from the onset. But while we can sheet some of the blame to McLaren, and their insistence that Honda show up in 2015 with their PU, I blame Honda's brass the most, for agreeing to such an impossible engineering schedule.