*grumpy hat on*
Our ACS 5.4 never has a bad day, no matter how much traffic we throw at it, kick it around with config changes, run reports all day long, log in with different browsers (never a problem with the html rendering) - we never need to worry about because it keeps on ticking like a Swiss watch. This is a quality product.
ISE on the other hand is like playing with dynamite. It's unstable a lot of the time, and will eventually blow up in your face, without much prompting or effort required. ISE 2.x feels like a beta product that needs special attention every day to resolve some new issue. It's not as if we're doing anything fancy with it - it just doesn't feel like a solid product from a major player like Cisco. I can see why so many customers have resisted moving from ACS to ISE. It's not that everyone doesn't want BYOD etc. - it's pure reliability. ACS is the rock you can depend on. I wish that for ISE too ...
It would be nice if the foundations were made solid and reliable so that we can all sleep well at night.