I cannot trace the article but I recall reading around Nov 2017 that Honda was surprised by the amount of data provided by STR and impressed by the STR's open attitude. Also in Jan edition of f1sokuho there was Asaki's quote that how extremely cooperative STR is.gandharva wrote: ↑Thu Dec 06, 2018 9:52 am"We have a very good collaboration. If we find anything that can be improved, we sit down together, discuss it and try to find a solution. When we had our first meeting with Honda last year I was shocked: they weren't aware of things about the single-seater, I mean data that also affected the engine, which meant that they hadn't been informed. Today we have a very good relationship and I am convinced that next year they will be able to win races with Red Bull Racing".
That's one sided view, Hasegawa was talking how generous McLaren is and how profound their capability is, McLaren lent a hand to Honda a lot in every aspects, battery was joint development, many components were arranged through/by McLaren, intake, plenum chamber etc were manufactured by McLaren etc and they cannot thank McLaren enough for that and so on. For 2018 they could not rely on McLaren anymore so increasing the rate of internal manufacturing as well as establishing new/different supply network was one huge challenge to Honda. At the same time Yamamoto assessed that it's lack of genuine exchange between the two parties and there was no intense and tangible discussions, that's because they sort of respected each other too much and just concentrated on the assigned job hence working methodology got compartmentalized and confined to each rather than interactive and mutual, McLaren expected high for Honda but after all Honda was not at McLaren's level and McLaren gradually got to lose patience and prioritize their way, eventually the relationship went funny. (these are quotes from Oct 2017 paraphrased)
So it's a matter of matching and timing, they both did their own best but things didnt work, of course both sides couldve done better but in such case what's in the best interests for both is to part ways.