The risk of Exroot failure is not that the router would not boot, but that it will not be configured to the same as before. The router would basically be offline, while it takes you time and effort to fully recover.
The backup of settings that can be done in LuCi only includes only /etc files and does not include related files that may be in other directories. In particular, packages that have been installed after the stock firmware or have been upgraded will not be in the LuCI backup and all of these have to reinstalled, assuming you kept records of them and any related data files.
Each owner can personally evaluate the risk-benefit and proceed accordingly. In the situation above, AdGuardHome was moved because it is quite large and there will only be 1 package to recover if the USB drive fails. In practice, the 64MB internal storage is quite a lot and 26% of it was freed up as a result, which leaves lots of plentiful space. I don’t think a multi-GB USB drive is necessary for this purpose.
Instead of and in spite of all the effort, I would prefer that GL.iNet build all new routers with 128MB or more internal storage, which both my GL-AR750S and GL-MV1000W have.