My dd12 was thinking about applying to Exeter for next fall's 9th grade year herself, but wound up passing for now. She's just one grade accelerated. The main reason she didn't apply was b/c I left the process entirely to her to plan and get done and she didn't get the registration process for the SSAT or ISEE together soon enough. The exams aren't offered really locally to us and the only one she could have done would have involved my driving her many hours and her having little to no time to study.
I figured that, if she was going to be considering that high pressure of a school that far from home where she'd be on her own to deal with school issues, she'd better be able to make it happen without significant support from me.
The quantity of work was a concern for me as well b/c dd, while capable of much higher level work than she's done for most of her educational career, is not a fast kid. She had a 504 earlier for SPD and dyspraxia that allowed her extra time, if needed.
All that said, the SSAT purports to be more of an ability/aptitude test than the ISEE from what I understood when we were looking at it. I was leaning toward thinking that the SSAT would be the way to go for that reason. I believe that the ISEE says that it has a reasoning or ability section that is 1/3 of the test. The other parts are 1/3 math & reading achievement and 1/3 essay writing.
