I think they worded that backwards and are referring to the adage (or maybe that is what the banks go off of?) that your loan shouldn’t be for more than 3x your income. So if you make 80k per year you can generally afford a $240k house.

Going above that 3x means too much of your income goes to paying for the house and you don’t have enough for other living expenses+maintaining the house.

Presumably the cert would be a smart card (similar to credit card chips) protected with a pin. And they can use revocation lists to remove cards that are reported stolen.

There would have to be a serial number at the least but that would change every time your card expired, and the government would certainly know who is issued what serial.

Another downside is users would need smart card or NFS readers to use them. Smart cards have been around for digital identification for decades now, it’s really surprising that more government haven’t pushed their use. From a user perspective though it would be pretty quick that every online service would start requiring them and any online anonymity would erode pretty quickly.