Yes, I used the "Tap To Pay" option while necessary, Watty... and it is a useful back up option, I guess. However, it doesn't provide a Chip and PIN option so you'd have to resort to sending a payment link to circumvent that where required, I guess. I've also heard more technically minded users complain that standard consumer mobile phones don't have the same security features as a dedicated card terminal ought to have.
I keep it in the centre console box, MfH... and always have done. I did come across a suggestion from an internet poster who successfully challenged them in similar circumstances - receiving a refund after threatening a law suit (probably in the US) - that he was advised during an exchange with Sumup support that not charging the yoke regularly (whether it needs it or not) could be a contributory factor. That's something I'd certainly be "guilty" of having never considered that it might need charging unless it actually needed charging... and it has been quite idle for much of the last few months on account of the good weather.