You are, of course, assuming the TX buffer is empty. What if you only want to send 3 characters (say, the number 200), yet the buffer, because of what you are already sending elsewhere, only has 2 characters remaining in it? Or worse, non?
For example, your ISR triggers while the sketch is already waiting for room in the serial buffer. Oops - crash and burn...
Remember: serial is SLOW - so slow your sketch may be spending a lot of its time waiting on that serial buffer to get some room in it.
So all in all, using Serial inside an ISR is bad, no matter which way you look at it.