Maybe I’m not imaginative enough, but I don’t see how you could brick a device. When you put it into the loader mode, you can rewrite any part of the flash (with those st tools, not MFDeploy). So, you can easily undo any bad tinybooter or firmware just by erasing the flash and trying again.
There are a few things I can think of to permanently damage an STM32. Like a poorly written internal Flash library that rewrites the flash very often, wearing it out quickly. Also, there are some registers, I believe, that will lock the chip from further rewriting of the flash. That could be bad (but nearly impossible to do accidentally).