I second this. I remember reading an interview with one of the mullvad staff saying "y dream openVPN config would be [insert more or less verbatim the wireguard spec], but due to the nature of openVPN that either won't happen or will take a long time". They were quick at adopting wireguard :)

They are very knowledgeable and spend a lot of effort in securing their VPN. I contacted them about what I thought was a bug, and got a very thorough answer about why mullvad behaved like that, what kind of threat it mitigated, and how I got the behaviour I wanted together with things I could do to keep safe (which was to update the kernel). Within 6 hours.

I am a very happy customer.