the most common idiomatic language is phrases like "time flies" not expressions like "it's raining cats and dogs", the first is particularly dangerous in translation

Actually 'Time flies' presents no particular problem for Esperanto.

Not only is flugi reasonably used figuratively in Esperanto, but also NPIV records (meaning 4) Iri tre rapide.

In Esperanto you can choose, depending on the required nuance, to say either La tempo flugas or La tempo fuĝas.

Unu el miaj amikoj estas stevardo de aviadilo. Kiam li ne respondas al la telefono, lia mesaĝo estas: La tempo flugas, kaj ankaŭ mi!

When I think of idioms, I think of phrases whose meanings simply cannot be divined from the words used. The "make" terms are nearly all idioms, for example; make up, make out, make off with, etc. If you don't already know what they mean, the words that make them up* are of absolutely no help; there is nothing either makey or uppy about make up, in any of its multiple meanings. To me, that's an idiom. A complete divorcing of meaning from the words used to convey it. Not what we encourage in Esperantujo.

Metaphorical statements, colourful speech (as sudanglo spells it), similes, and so on, where, maybe with some thought and the right context it's likely the listener can figure out what's meant, I think those sorts of statements are fine in Esperanto. Those sorts of things aren't really idioms - the words used do clue the listener in, even if they have to think a bit and get their abstract on a little.

So, I just ask myself "do I think a person of reasonable intelligence could figure out what I am about to say?", and if the answer is "probably", I'll go for it. And if they misunderstand and we get in a fight because of it, well, we can always faras supren* later.

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* I couldn't help myself. Sorry.