So what is the bottom line? Use this portal or not?
if the issue is only price, then as SP03 pointed out, C1 has a price match guarantee. however, the first problem is the requirements and there are many situations where it doesnt cover
The itinerary has to be an exact match to the booking you made on Capital One Travel.
You have to submit your claim within 24 hours of making your original reservation.
The price of the new itinerary must be available to the general public at the time of review without coupons, promotions or member discounts.
The price has to be in U.S. dollars, including all taxes and fees.
the second problem is you get back c1 credits. this makes you kinda stuck with the portal; if you make use of the price match guarantee with every booking, then you will always have credits leftover on the portal
furthermore, the issue isnt only limited to price. problems can surface when booking via 3rd party ota. this is especially true when you need to cancel or make changes. so it depends on how much you value those extra points, and if that is worth paying more sometimes, as well as the time and effort to resolve problems that arises from 3rd party booking
for me, the exception is when 3rd party booking is a lot cheaper. e.g. i recently made hotel bookings through citi and chase because, in total, they were about $150 or 20% cheaper than booking direct. c1 had similar prices to citi and chase, but i booked with them because i had credits