Experience is a good guide.

The main issue is usually latency.  Assume for a moment a system with
one or more uarts.  When a serial character comes in, can you poll the
uart in time, before the next char arrives?
Can you GUARANTEE that you will?   With interrupts, you can know that
it will be a very short time.  The sticky thing is when you have
multiple interrupt routines, and if they can take some significant
time to execute.  It's always a very good idea to have your interrupt
routines do as little as possible, letting the main line code do the
bulk of the work.
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