Technically is *should* be fine as long as you can find a method of cleaning and sanitizing that does a good job.
But here is the very important part:
The mash tun is PRE boil, so your level of clean/sanitizing can be MUCH lower than that for POST boil equipment. I just run hose water through my manifold and rinse out the mash tun, wiping the sides with a washrag to wipe the stuff off. That's it.
So my opinion is, if you like the way it WORKS, then it's fine.