I don't have any actual stats, but I would guess that nearly all Millenicom customers are mostly happy customers just as nearly all Verizon data users are mostly happy customers. This is one of only a few complaints I have seen about Millenicom and folks having problems love to post their issues on forums, here or otherwise. Depending upon location, service can be terrific or terrible or anywhere in between. This can even vary day to day and hour to hour depending upon the number of users. As an example, go to Quartzsite, AZ in June and you will have exceedingly fast Verizon service. Go to the exact same spot in the middle of January when 100,000 RVers descend upon Quartzsite and try to do much else than sending and receiving your e-mails!
I do not mean to diminish the problems reported by the OP and I am seriously sorry to hear that you had such troubles with the service. It does sound like Millenicom tried but failed to solve the problem and then provided at least a somewhat reasonable refund and resolution.
I have no affiliation with this company but have used their service without issue in the past. I now use Verizon directly only because I no longer use more than 6-10GB and the Millenicom plan really only makes sense if you need most of that 20GB/month.