I just discovered that my recent upgrade to 3.3 broke certain aspects of our site. Before finding this discussion I figured out the problem and uninstalled Pioneer though and I unfortunately didn't copy the error message.
A bit of info first:
Moodle 3.3.2 running on an Ubuntu 14.04 server using Php 5.6.31 and Apache 2.4
I was aware that Pioneer wasn't very friendly with 3.2 but most of it was working and it hadn't had an obvious negative impact on other areas of our site. After the upgrade, I found that I could no longer edit user profiles, change theme settings or open a certain course. (That should have been the hint as I knew it was forcing Pioneer, but the user profile thing threw me off.)
Turning on PHP Debugging and Show Errors, I noticed even editing a user profile was requesting an element from Pioneer. Uninstalling it from the Plugins Overview page fixed the errors and now everything seems to be back to normal.
I agree with the recommendation of the OP: Pioneer should be removed. I read somewhere they were going to try to build a similar theme based on the new Boost theme in order to get full functionality, but don't think they've gotten round to it yet.