Conceptually they had the core of a very good story, which was the idea I could have "Facebook where I segregated the people in my lives". It was even quite good timing, as it was mooted around the time people were starting to bubble up stories about "my boss started reading my Facebook and I got fired", "my family are upset by the politics my friends discuss" and so on.
The biggest problems with G+ from my perspective, around launch time, was other than Facebook's network effect, that the dribbling out of invites completely cut against the core success model for a social network, and the whole circles functionality ended up being one of those UI nightmares you get from Googlers who don't understand how normal humans actually work.
G+ positioned itself against Facebook sort of like DuckDuckGo went against Google: we made the same product, but fixed X!!!, where X is some gripe about the incumbent's product that only a small percentage of the product's potential userbase cares about (privacy, in both of these cases).
That was (and is!) a fantastic strategy for DDG, for whom a fraction of a percent of all search traffic counts as massive, life-changing success. Google is not DDG, though. G+ would have needed a much larger share of the social networking market to be considered a win for Google, and the initial differentiation was not anywhere near clear enough to get there against a rival as strong as Facebook.
I also agree about the UI mess, and all that, of course, it was not a great product to use out of the gate.
Facebook annoyed many users each time a new update came. The privacy settings were constantly being reset. I do not believe facebook has a great UI to this day. Google+ is not great but it does not offend me the same way.
And I say that as someone that doesn't have Facebook account.
> The privacy settings were constantly being reset.
True. What privacy-minded Facebook critics often miss is that privacy settings seem to have been constantly reset towards "more private" mode. Hell, at this point Facebook is starting to get really annoying with those recurring reminders about visibility of my post. I post all-public because I like it that way, I hate the constant nagging about switching to more private mode.
To chalk it all up to network effects is to let them off the hook too easily, after all there is no reason you can't build other successful social networks concurrently with Facebook, you have your Twitters, Instagrams, Snapchats, etc.
The way I see G+ failing is because there was no soul or vision to the product, it was driven by a simple fear of Facebook, and it's implementation was a simple conglomeration of features that constituted a cool tech demo but was not shaped by real users or a real use case (Google does this often, see Wave, but at least in that case they were trying something novel). In short, G+ was not true to Google's DNA.
I think my view still supports your main point though—stacks simply are not the same for each company. Lower level stacks tend to be more similar, but at the top they are serving unique market segments, so they are simply not fungible.