First, Google has moved beyond killing third party apps 'terminal' apps like Plus to killing support apps like Cloud Print. That alone means that as a user I'm scared to rely on integration-level apps.
Second, the rate of turnover is so high that it actually causes me usability problems pretty often. Everyone hates Microsoft Office redesigns, but they're infrequent enough to get used to. Between dead apps, changing permissions and features, and constantly being thrown in and out A/B tests, I basically never feel at ease with Google products. If I have to look for instructions on how to do something, they're almost always unusably out of date.
(Storytime: the incredibly useful "arrive/depart at" feature in Maps vanished from my mobile app for a while. I got into a discussion about it with another programmer who missed the feature and a Google engineer who thought we were just looking in the wrong place. It quickly became clear that fragmentation or long-term A/B testing meant each of us had vastly different options and interfaces.)
Third, killing third party ecosystems catches up in the long run. Normally that comes when companies lock down services, but here it might come from uncertainty. if developers shy away from Google products and I can't trust trust Google-official services to say alive, how long before I look for more robust options? (Answer: I already am, increasingly often.)