Yeah, I wish there were numbers/trends, published somewhere. I kind of wonder if this is relatively isolated, or if it's more of an "eventual" issue, for most. I really hope it's the former.
It could be, like Krystianpants aluded to, a side-effect of bad charging due to OS issues, in Insider builds, it's certainly not outside the realm of possibility.
The thing along these lines that really makes me go "hmmm..." sometimes, is that I'll put my phone on charge, and it'll go nowhere for a long time (maybe a few percent in an hour), but I disconnect it, and reconnect it, and boom, I go from say 86%->100%. Then I reboot, and I'm back to say 88%, and charging for awhile again, to get back to 100%.
This is concerning, which of the estimates above is correct, and in which cases is the estimate way-off, causing the OS to potentially over-charge the battery?
The battery is supposed to have heat-sensors and the like, but I'm betting there are ways for the OS/firmware to ignore these, within boundaries, and continue to charge, at least for some period, which would lead us full-circle.
I'm usually on at least Release Preview, I think we need to be active beta-testers, if possible, to move things forward, but I've never had cause to think this might incur physical device damage...