Over time the cannisters of any of 'em can lose propellant. The good ones will have expiration date printed and readily visible. I've seen a few on shelves for sale where expiration date had already passed!
Another "caution" is to look into rules and regs. for specific university. Some may actually inhibit/prohibit as "dangerous weapons" or some such ---------. Your wife might even be able to more "legally" carry a pistola in car on campus than to have OC in a classroom ... "The large print giveth and the small print taketh away."
I could not get the link on "keychain" to load. But if it is the Kubaton that has the little OC insert and semi-resembles a little maglight, I know a guy
who's daughter is equipped with one and carries it daily on an unnamed campus with some of the aforementioned unsavory rules.