I was considering running down the balance left on my Oyster▸ and relying on contactless alone.
However, if you did a "few" contactless journeys without a chip and PIN transaction I assume there is a chance the card would not work which would be inconvenient. Obviously the banks do not elaborate as to the number of sequential contacless transactions that can be made before that happens.
I used my contactless card for a few weeks on the tube and encountered no issues. By touching the card on the reader it doesn't actually authorise a payment in the traditional sense, instead noting where the card is touched during the day.
TFL▸ then do the maths and decide how much to charge you at the end of the day (as I understand it!). The payments only ever appeared on my statement as one daily amount and sometimes didn't appear if I reached the weekly cap.