Its funny, I will use the capitalone shopping portal to check if there are any good discounts when I know I will be making decent size purchases or a lot of travel bookings, but I have refused to install their browser extension for data tracking reasons. Which is stupid because google probably knows when I take a shit every morning.
The one thing I have noticed is that actually receiving the cashback is sporadic and its too much of a pain in the ass to argue for it. An example would be when I take 3-4 week road trips, I often will book my lodging throughout the trip on the same day or the day before using priceline and/or VRBO, both of which offer cashback at various levels through capitalone. only about half the time does the credit hit. I am just too lazy to document and try to recoup it. And occasionally after 6 months I will get notification of a credit for that trip. Its not the worlds most foolproof system they set up.