Hell, I typed a lengthy and detailed reply to your post and by the time I clicked 'Post' the blasted forum had logged me off.

In a nutshell though the position is this:

In order to use ISRs of any length, short or long, you need to take into account exactly the same things. You cannot simply say 'they must be short' because you don't know how short unless you have actually worked it out. If you do simply choose some arbitrary execution time limit you are putting yourself in the precise position you advise against - you have code that may work this week, may pass all the tests then fail next week.

It is utterly useless to have some sort of general 'ISRs must be short' rule.