Seconded. It’s not a bad idea, but it’s another point of failure for sure.
To add on - several of those USB controllers implement stuff in non-standard ways, so if your board fails you either need another one of the exact same model and firmware to read the data off those drives again. It’s very likely if you just bought another DAS/USB controller that the drives/partitions would be unreadable, and you’d have to start over again.
OP, it’s not a bad idea unless you don’t have a backup plan. If you are planning on having no backups, then this is much more risky than just an external hard drive, this could very well lead to complete data loss. If you have a solid backup plan, then go for it! If you don’t, then for the love of god you aren’t ready for it.
Honestly, if you only need a few bays, just look at getting a Synology.
I mean it’s up to you, your decision. In my experience going the usb route though only leads to more cost later, to me it’s better to just save your money and go with a solid solution in a couple of months. You don’t have to go crazy. An old desktop with some extra sata ports is a fine start