Besides having to pay for G+ (where FP+ was free), the biggest downside for us is having to start each day at 7 a.m. with a plan A and a plan B, and sometimes a plan C. Because what you wish you would get--well, it is called Genie--and what you will get might be vastly different. If you don't have must-do attractions and are just willing to go here or there, depending on what's available, and don't mind going from one end of any park to the other and back again, then G+ can work pretty well. Also, it works well if you're in a park from open until close. And additionally it can work extremely well if you have only one or two attractions that you'll use G+ for.
The second biggest downside, to me, is that once you book your G+ selection, you can't edit it. If you want to change things, you have to cancel the selection you've made and hope that the one you saw that was an improvement will still be there when you can get back to it. FP+ could be modified if there were other times available.