I have just started checking out Photos for OS X. The absence of star ratings in Photos is definitely a pain. However, I find that rating photos via keywords is not as bad as I had originally expected. If 1 Star, 2 Star, 3 Star, 4 Star and 5 Star keywords are created, with the shortcut keys 1, 2, 3, 4, and 5, respectively, it is quick to add or change ratings when the Keyword Manager is open. Pressing a number key adds the rating. To change a rating, press the number key corresponding to the existing rating to remove it, then press the number key for the revised rating.
I go live every Wednesday at 10am Pacific for a show of random chattering and meandering tech. Get any lingering questions you have answered!