Users vs. New Users
In my report I saw that I have less users (-4.18%) but more Sessions (+8.5%) and less New Users (-10.32%). All filtered to Device Category = Mobile.
One natural way to interpret this is, that we have less users which interact more often with the page. But this assumption is not backed by any other data.
My question is, could there be another reason for that. E.g. another way Google counts the Visitors?
Re: Users vs. New Users
That could be one way to interpret or users are accessing your site via other devices instead of mobile.
You should also keep in mind that there are some things that can affect this e.g. a coding change that impacts the cookie domain or account and property to which the data is sent would make all users initially "new users" which of course any comparisons that included the change data would show as down. This would also skew your campaign data.
Other factors could be viral traffic, referral spam or a drop in email promotion as more and more this is mobile with less than normal page depth in session.
The bottom line is that GA has not changed how users are counted - it's tied to the presence of a _ga cookie (for Universal Analytics) that is accessible to GA and matches the account and property index.
Hope that helps.