The app can be re-registered as many times as necessary, and it's free assuming you already have a smart phone.
The physical keyfobs last forever*, and my older one from when they were first introduced for FFXI has been through the wash at least once and still works to this day. However; they aren't free*, and once you deactivate it the serial code is locked out permanently and can never been registered to any SE account ever again.
I've never had an operational issue with the app outside of the site giving me grief in registering it, possibly just a brief net issue, but aside from that I've never had an issue with the token or the app being unreliable after registration.
*I'm sure that's an exaggeration, but given my first one is as old as the SE Token system itself and is still kicking, battery life has yet to prove to be an issue.
*Naturally there are exceptions with CE's or promotional offers, I'm sure.
