Tbf I'm not concerned (should I be?) I've not owned a phone since 1995 and this current S8 thing seems to be great on the battery, the Mrs phone is <£200 and she uses her bedside one (5.4v) to charge 99 percent of the time anyway.
Does the additional 2v really cause that much issue?
We have a couple of Galaxy Tabs and a kindle that often get charged on it and they are fine tbf.