The whole idea of the ITSO standard is that the cards and services should be interchangeable (in theory).
But at the moment, just focussing oh Go-Ahead's "the key", you can't use one of Brighton & Hove Buses smartcards (also branded "the key") on Southern Railway. The other way round, you can't load bus only products onto Southern's smartcard but you can load PlusBus in addition to a rail ticket which can be used on Go-Ahead's Brighton & Hove Buses and Metrobus .... but only those operating in Crawley and Redhill, not their red London buses. Confused? I certainly am writing this.
To add to the matter, London Midland's "the key" also doesn't work with Southern's system yet. In addition, there is no break of journey without being manually let out of the station (but I have it on very good authority this is being implemented very soon).
Stagecoach have also got their "Stagecoach Smart" implementation. But this isn't compatible with "the key" ... yet. Again, I have it on very good authority (from the same manager) that this is being worked on to cover the Havant/Portsmouth to Southampton line. Even East Midlands Trains "Stagecoach Smart" won't work with South West Trains, or even Stagecoach buses. Which means I currently have three "Stagecoach Smarts", all with identical designs which are for use with different products. At least Go-Ahead have put different designs on their cards....
And now there are even paper ITSO smartcards. The new design of Glasgow Subway tickets are slightly thicker than normal rail tickets and contain a microprocessor and antenna. These must have come down in price considerably as a single ticket only cost ^1.50. I understand
TfL» looked into paper
Oyster▸ cards for visitors at one point.
Anyway, the point of this post (which probably sounds more like a rant) was basically to highlight the poor integration between different ITSO implementations. These are only the ones I've used/attempted to use. I now have seven plastic ITSO cards plus a few disposable paper cards. I don't even know where I've put some of the plastic ones.