I like to have two current accounts running, so that one bank's systems falling over doesn't cause me to be stuck with no access to money. When I was cycling D2D in 2018, TSB's system went down and locked everyone out. I think you could still use the cards at the time, but I'm not sure. I wouldn't have liked to have been stuck mid-tour without access.
Starling works well. Revolut are another I've tried which has also been fine IME.
With the Vivoactive, at least, you need to enter a PIN before it will do anything, so in some ways it's more secure (and less convenient) than a normal contactless card. IIRC, the 'card' then stays unlocked until the watch can't find a heartbeat, (which hopefully means you've taken it off rather than you're heart has stopped
)