the "not knowing your return time" is definitely a negative - and basically the same negative as the old paper FP (e.g., FP-) system (though with that you do have the standby option - but even that, you won't necessarily know ahead of time how long the line will be at the time you arrive)
Good thing is the 2 hour return window so you can go off and do other things, even go back to you hotel or go eat or whatever as you have up to 2 hours to get back to the park.
No system would be perfect for everyone, but I think, especially given the fears of technical issues, this is probably the best they could do for the most people