If you're seriously concerned, why not use an external drive of some sort and install Yosemite to it? (Bonus points: clone your internal disk to it first, and upgrade that.) Boot it up, test out your critical functions, and then update your main volume if it works out for you.
You need to do the Bonus points version of your recommendation. Running an OS off an external will just make everything feel sluggish (unless you have a lightning enclosure and spare SSD laying around).
That would depend on which interface and drive technologies you use. A SSD connected via Thunderbolt would likely be faster than an internal hard drive.
