My only contribution is if you want to eventually go to Shimano Di2, then newer frames tend to be designed for internal cable routing, which makes the electronic group installation easier. Older frames might not have an easy method to get the E cables internally routed, though you can do external battery and cabling. And you do have an option to go SRAM wireless, which is pricier.

One advantage though would be that older frames might have a BSA bottom bracket shell, which IMO is a whole lot easier to deal with if doing a home build up. Then there's the trend to 1-1/8x1-1/2 fork steerer tubes and headsets on new carbon which is can be a PITA to find headsets for, but possibly a good design in terms of carbon steerer longevity.

If you are doing a mechanical build-up with Ultegra or 105 level, then they are good deals and R&A is a good source, as is Adrenaline as well as GVH in Oregon.