I've had several cats with hyperthyroidism, and for various reasons each time chose not to go with this treatment. I've not heard of a cat dying from the treatment, and think there are real positive aspects to this choice (though I'm not a vet or vet tech so can not comment in any way except anecdotally). What I have heard of a few times on the list I am on for folks with cats who have Chronic Renal Failure (CRF) is that sometimes hyperthyroidism is masking CRF. It doesn't show up on the tests done prior to the procedure, but several months down the road, your older cat is diagnosed with crf.
My best suggestion is to do a google search on hypert and crf to see if you can pull up an article on this, and to read over the feline crf site (aka the avatar site) which I believe comments on cats with hypert developing crf. A google search on feline crf information should bring up the avatar site.
good luck with your kitty,