By “under the floor a/c”, I assume you mean a raised floor with the cold air underneath and supplying the computer room via grates and perforated floor tiles?
If so, a few of the advantages are:
(1) if you want to install 60′s/70′s era mainframes, you’ll be all set.
(2) Your data center will fit the model of what most people think of a computer room should look like.
(3) There is a tremendous amount of expertise available to design & build them.
(4) You’ll be helping to support the economy by purchasing plentum-rated cabling, underfloor fire-detection and suppression systems, floor-cutout sealing grommets, perf tiles and/or grates, 50-year-old CRAC designs, floor support systems, higher construction costs, etc…
(5) You can learn to cut cable-cutouts in floor-tiles, and learn to move tiles around as cooling requirements change.
(6) You can proudly point out you’ve built a “Modern Legacy Data Center”.
(7) You won’t have to explain why you don’t have underfloor cooling, or in some cases — even a raised floor at all.