I have been holding off on advancing to 10.10 having been "spooked" by this low "WiFi issues" thread.
My employer required me to "replenish" to a new Mac, and I selected the Macbook Air running 10.10.5. In comparison to the other Macs I use running 10.6.8, 10.7, 10.8, 10.9, this one running 10.10.5 is the best from a WiFi perspective. At home, we get cable internet at 60 Mbps and it is solid on 10.10.5. Notably, in one far corner of the residence where my router signal is very weak, weaker than my neighbor's offending WiFi signal, internet was before ~ only 5 Mbps based on internet Speedtest and used to sometimes spontaneously disconnect, but is now ~ 15 Mbps (Speedtest result) and never disconnects. Closer to the router our internet speed is the full 60 Mbps and the WiFi itself goes up to about 130 Mbps which is about all this router can provide. In this sense, for me 10.10.5 is more robust. At work I get both 2 and 5 GHz connections and > 200 Mbps depending on physical location, as the access points there are faster than my home router.
I asked the IT technician at work (we have several thousand Mac laptop users here) who delivered my new laptop if there were more problems with wireless on Yosemite; he replied no, he felt there were less. One thing I have noticed is that sometimes Macbook Pro with 10.7 spontaneously loses the WiFi and required turning WiFi off and on a few times or in extreme cases a reboot to recover, but with 10.10.5 I have had zero such disconnects.
That said, where there is smoke there is usually fire, and I suspect what is going on with those reporting problems is either WiFi interference (perhaps 10.10 is more sensitive to that than earlier OS, which if true would be unfortunate), or setup issues/conflicts with routers, or other setup/conflict issues, or a hardware problem. Why did these problems just surface with 10.10? Well, if one looks back in any of the earlier OS discussions (10.9, 10.8, 10.7, 10.6 etc), one sees that with each new OS introduced, there are some who report good WiFi has now become bad. Any system change or change in WiFi protocol (this did happen with 10.10) can trigger a problem with something that was maybe was working well but was possibly marginal.