I use Mullvad for 2 years and yeah it's been a good VPN. Global outage have been very rare, maybe it happened 2 or 3 times altogether. It happens however that some websites are blocking Mullvad servers, usually, it's just about switching to another server to get this working.

The desktop client also supports some obfuscation schemes (UDP over TCP) which is useful when you're in countries which block any kind of VPN. The default smartphone app doesn't support this out of the box, but they have some tutorials to setup Shadowsocks and OpenVPN to route the traffic over https as well