When do you have your wallet with you but not your phone?
Good point! I was thinking last night that I left that out of my list above 
Yoyo manages brightness itself which is pretty slick. I’ve never had a problem with it; fastest is with their own hardware - face-up phone-shaped scanners mounted next to the till, works really well. One of the places using it at my university just use their phones (another thing I missed - sending money to others, though I think that’s coming to Monzo too).
When I go out shopping or for a walk in my town and only take it if I am going out for a few hours or more further afield, otherwise leave it home plugged in to a charger as otherwise it won’t last a whole day from 08:00-01:00
I’ve used other apps that control my phone’s brightness in order to scan a code, StoCard included, but they all suffer the same problem. It’s not the fault of the app that the screen is so reflective.
Or that many merchants don’t realise that they need to have specific types of scanners to read codes from LCD displays. 
I use YoYo 3-4 times a day during the week, it always works instantly. Unsure what their secret is.
Just enforce compatible scanners in the requirements. My issue is mostly with merchants having their own loyalty systems (Tesco trying to scan on-screen clubcards using the laser scanner rather than the optical handheld one) or where people try and put incompatible cards in an app like StoCard and wonder why they don’t ever scan when all the merchant has are laser scanners.
Just adding this here for reference as this topic is now starting to touch more and more onto what I’m discussing here:
As part of my job, I’ve just started looking into best types of barcode scanners to be able to read from a phone screen, in this case related to ticketing. It seems some people have problems with it and others don’t.
I personally never have a problem scanning Barcodes/QR Codes from my phone screen.
Simple example; I use the ATMTicket Machines at Cineworld about twice a week, and I’ve never had trouble scanning them.
Currently Nexus 6P, and previously on a HTC One.
I’ve also seen YoYo integrations in a few places, it has it’s benefits and is extremely quick, but I still prefer the idea of Apple/Android pay being instant without even a screen unlock sometimes.
If you want a list of where currently uses it, the app lists all of the outlets…
It’s generally 1D laser scanners (such as the omnidirectional types installed in supermarket (self-)checkouts) or used in warehouses that will always fail. If it emits a perfect, straight thin line (even if it’s waving all over the place), it’s a laser scanner.
1D CCD scanners use LEDs and are generally much better but not perfect, you’d have to test each model at the very least.
Almost anything that scans 2D barcodes through some kind of camera based system will be fine for reading from LCD displays.
(I JUST REALLY LIKE BARCODE SCANNERS, OKAY?)
I think it’s important to set aside the consumer perspective (which we disagree on) and consider it from that of the retailer.
Yoyo appeals to retailers because it makes them look savvy, they get detailed analytics, and they can offer rewards or loyalty schemes without building yet another in-house solution contributing to loyalty card/app fatigue.
Even if Monzo offered the same feature set, I think they’d be a lot less comfortable partnering with a bank in the same way, and at the very least would need to be open to doing the same with others.
Yoyo on the other hand, to reiterate, is linked to any debit/credit card to offer a bank-agnostic payment method. It competes with {Android, Apple}Pay, contactless cards, and Chip&PIN; not Monzo.