They look like they place an awful lot of load on the seatpost.
In truth I have’t carried a computer on one, and am wary of the ones with the drop down side pockets. The basic one holds a change of clothes and tools though.
Tubus Fly is the answer, then decide whether you want a bike with eyelets (alloy mainly) or are happy with the QR option,
If you want to take it off a lot, I think a seatpost collar is a better/faster option than the brake mounting. If your race bike has an aero post, then you will have no option. If your race bike has aero post and hidden V brakes, then you will use a rucksack 😉