Reliable is a flexible word. Hetzner does frequent, albeit often brief, maintenance operations that would incur a down time on a high-throughput live service. Not impossible to design around with enough redundancies, but far from "I have one FreeBSD box that's been up for 467 days with 100% uptime" that people used to brag about in the early 2000s. I have few VPSs and bare metal instances that I just `sudo reboot` when there is a kernel update with practically "zero" impact or downtime for myself. Something unimaginable to do in a production system where you need to cordon, traffic drain, update, reboot, uncordon then orchestrate that across an entire fleet of instances with different roles and characteristics in a specific order.

Not relevant for a self hosted mail server..