Later on - the more elderly Gaelic gentleman has very weird pronunciations - really catches one out! There seems to be extra syllables put in and words which don't sound at all like you would think they should (take his Ghàidhlig for example, it comes over as a Rahnich sound! )- I suppose it's a heavy dialect and one we must get used to - but it's not easy. Like you say - I also have to replay the sound several times before making a wild guess!
It's just as much the fact that sounds are often difficult to distinguish in real life. Duolingo is an unnatural context. It is almost impossible to imagine a context in real life where you would not be able to tell them apart from context. Just try.
Tone of voice would also be quite useful. You are chatting to your girlfriend about your niece. 'Tha ??? cho brèagha', you say. Later you look into her eyes and say 'Tha ??? cho brèagha'. I think she would understand the difference by tone of voice. She would probably get the gist of the second one if you were speaking Croatian. D
In many languages, u is problematic as there is one letter that represents both a broad vowel and a slender one, and they are somewhat interchangeable.
Look at English sugar where the s must be slender as it is pronounced as Gaelic siùcar.
Or super with a broad u, which is slenderized in the German über, where the dots mean the u has been slenderized, or the same word in Greek, hyper, where the y is the Roman letter that is used to show that the Greek u is slender - and ends up being pronounced as an i in English.
In Welsh, du (Gaelic dubh) is pronounced virtually the same as di in some dialects. Di in turn is the Welsh for thu (lenited form).
In short, u does often have a bigger than normal range of sounds, with i at one end of that range. Since the th is silent (for no reason that I know of) then it is quite understandable that these two words are difficult to distinguish in some dialects, especially unfamiliar ones. D
Revisiting this section after a while and when writing "brèagha" I'd forgotten the accent but hadn't been marked down or had that pointed out. So for this one I selected the one without an accent and it's wrong (of course). Is it possible to have it pointed out when the accent is or isn't there in other questions? I'm sure it used to do that, but isn't now...
Duolingo is at best temperamental at pointing out accents. However, there have been a number of bugs introduced at the most recent 'upgrade' so things could have got worse. It would be interesting to see if other people report consistent failure to mention accent errors. D