My $0.02 on the various replies and the OP......
USB sticks don't really "wear out", but they are definitely more susceptible to corruption which may or may not render them unusable.
From personal experience, I found running them on my router resulted in them getting quite warm. Not saying this is bad, or that the use of the stick itself was the cause vs it acting like a heatsink, so it could actually be a good thing.
On the positive side, USB sticks (thumb drives or whatever the phrase of the month is) are pretty cheap these days. I bought a box of 64GB sticks for under $10 each to basically have as giveaway media to transfer stuff to friends. This makes it easy to have spares and "clones" ready to swap in if there is a failure.
On the negative side, performance is often not great. Particularly if you have many files or lots of small files. It really depends on what you are running and using it for. There are a number of posts about router CPU pegging at a high rate, that was attributed to the USB usage, indexing, error recovery, slow speed etc.
There are MANY free utilities to read and write USB sticks to / from image files so it is easy to create backups, although as mentioned already, it is usually just as easy to copy the files to another location, preferably using rsync (or similar) or as a cron task, etc
There are many people running heavy duty functions all day every day and never see issues.
Personally, I wouldn't run anything beyond some simple scripting from a stick. Although at first I thought it was amazing to be able to run all this "stuff" off my router was great (diversion, samba, and many others) I found I was loading the router, the key single point of connectivity (and potential failure) with things that really had no need to run on the router.
My AX88U runs typically with less than 10% utilization on a single core and that's the way I want it to stay. For me it made sense to put the VPN services on there, but nothing else. Raspberry Pis (or similar super cheap devices) make great DNS servers, Pi-Hole servers, web servers, etc. I like them in particular as I can run them multi-homed, with multiple network interfaces, each on a different network. If you want NAS functionality, get (or build) a "real" NAS, unless your requirements are minimal.
Ultimately, the choice is yours, and it really is a choice. There are risks and rewards to the various hardware options and it pays to be flexible in what services run on which devices, and their placement in a network infrastructure.
Having single purpose (or at least limited purpose) devices also can mitigate issues with updates, compatibility, performance, reliability, etc. I have a 15 year old NAS that performs just as well today as it did when I bought it. This type of setup also can introduce more headaches, power requirements, physical space, wiring, etc.
