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I've been using iOS 15 for a month or more, and I've had no glaring issues aside from Exposure Notification using excessive battery usage.

Excessive battery usage IS a good example of a "glaring issue", especially for a mobile device.

On one specific service that is opt-in (and can be disabled at any time). Pretty important distinction.

This is most true, but I've also had the same issue with iOS 14.8 for a while between restarts. So this is not an iOS 15 exclusive.