I'm a professional documentary editor and often use FCPX 10 hours a day. I have about 64 terabytes of Thunderbolt RAID arrays on my 2015 iMac 27 holding many different projects. I am currently working on a documentary which has about 10 terabytes in a single library and will eventually have 20 terabytes.
I have never experienced FCPX deleting a project, whether during rendering or any other time. No other professional FCPX editor I know has experienced this and I've never heard anyone else even mention it on a forum. What the vlogger said about you can't back up manually is incorrect. Below a tutorial about how to do this.
Anyone who uses video editing software professionally knows it is all imperfect. I used Premiere for years and periodically had problems with it crashing repeatedly. Likewise I've had FCPX crash many times, but it's never deleted data or projects. In rare cases I've had to step back to a previous auto-saved project which FCPX makes by itself. At most this cost me a few minutes of work. If I have the slightest concern I will manually make a project snapshot (see below video) but this is virtually never needed. You need it more to safeguard against an editing mistake. In general both Premiere and FCPX are stable but you can encounter situations where they are unstable.
The vlogger may have some kind of system problem such as nearly full hard drive or corrupt media. If FCPX deleted projects all the time, why would anyone use it? There are over two million people using FCPX worldwide. It would be impossible for that huge user base (including many full time professional editors) to be unaware of FCPX deleting projects left and right.
MacBreak Studio - Faster manual saving of project snapshots in FCPX: