In general terms, it all depends. Good support systems is the key issue in cases like this.
One of my teachers was an American who had had a brain tumour, and ordaining in the US would have been out of the question, due to the need for ongoing medical care (though he was very mobile he still needed some attention). His (Thai monastic) teacher said if he wanted to ordain he'd have to come to Thailand, since as a monk in Thailand there was free health care and ample lay support.
Of course, that would not work in all cases, but did in the case of a well-connected Bangkok Wat.
[Unfortunately he died a few years ago, but that was caused by injuries sustained in a car crash, not the brain issues.]