So, I got my Starling card, and can confirm that Samsung pay works well with Starling. Just remember to turn on the ‘Mobile Wallet’ in their app before trying to setup Samsung pay (otherwise it will fail). Not tried it from phone, only from watch. At the moment I will run the two side-by-side, paying from the watch, and just leave the Starling card at home.
The Starling debit card is an odd creation. With its 16-digit number on the reverse rather than the front, and number+text just printed, not embossed like Monzo, it feels like one of those prepaid ‘voucher cards’. I know that some outfits will refuse non-embossed cards (eg for hire cars) as they are more easily copied.
Anyway Monzo, if you really are serious about growth then you will need to cover more bases. Android wear is all-but dead. The only Android-supporting smartwatch worth having comes from Samsung and will likely never support Android Pay - only Samsung Pay.