Hmm, metal straws...what could possibly go wrong?
Keeping them clean would be one issue. I have something that has a stainless tube, and it is very difficult to the the inner side of the tube clean without using a pipe cleaner.
As I mentioned, the ones we purchased from eBay came with a mini brush to clean the insides, and it’s no hassle to do this. I’m sure they’ll be available here too...
TBH, I’d be more worried about the design of so many water bottles, which appear almost deliberately designed as germ traps (those flexible plastic straws some use; the crevices no brush can reach...).