I have a theory. See what you think.
The Yosemite wireless function gets screwed up when switching between 802.11g/n and 802.11ac radios. Normally this switching between radios, based on signal strength etc, isn't an issue, but for whatever reason Yosemite can't handle this behavior and loses connection completely during the scan/swap.
As a test, I separated 802.11g/n and 802.11ac traffic into two separate wireless networks. Then connected my 2013 Macbook Air and 2013 Macbook Pro Retina to the 802.11ac one (selected from pulldown). So far so good. Obviously, this isn't ideal, but it might be much less annoying than dealing with constantly dropped WIFI in the short term until Apple releases a fix. I should mention that the problem was consistent on both laptops, even when not roaming between base stations.
I've seen similar problems with Mavericks on the newer 802.11ac Airport Extreme products when I used to use them.
My wireless equipment are Ubnt UniFi AP-AC units running 3.2.5 firmware.
I'll continue to monitor and update this thread if anything changes. If anyone could test the theory I'd appreciate it.
Rob