iVPN is a great choice in terms of security, they also use STboot, but I think you're just flying under the radar with their IPs because they struggle with the same problems as Mullvad.
Yes, it only works better because the obscurity to IP ratio is good. It could easily be as bad as mullvad if they became more popular. But as I understand it the really popular VPNs address this with huge pools of servers and IP cycling?
One other issue I had with Mullvad that put the nail in the coffin for me was randomly laggy DNS resolvers, they would get fixed just by the time I start investigating it, but it kept happening... I say this as a mostly happy user for probably 7 years, but then found myself having to turn it off more than on to be able to access most sites.