I think people say that virtualizing TrueNAS is not recommended is just because it's easier to make a mistake when setting up a VM than running it on bare metal. I've seen people passing virtual drives to TrueNAS instead of passing whole controller. I've seen people struggling with network setup (either passing a LAN port or joining a vBridge). Depending on the hardware, you can have problems with passing PCI-E devices, but that usually happens on a consumer boards, where for example IO-MMU implementation varies and you can end up with sub-optimal IO-MMU groups. All of those problems are gone when doing a bare metal install, where all the hardware is directly accessible. But most people just regurgitate opinions found in old posts without making any research for themselves. Virtualization made a huge advancement in the last decade and it's just wasteful to host one service on one machine, especially in a homelab environment, where space is limited and your wallet directly feels the electricity costs.