It only has a single vowel, which is an r-coloured vowel...which most languages don't have. For that matter, many languages don't even have our "r" sound, so colouring a vowel with "r" is incredibly hard when you don't even have that consonant to colour with!

Not to mention that after using that r-coloured vowel, you have a semi-syllabic L immediately afterwards. (Is squirrel one syllable or two? Depends on who you ask I guess!). As you may know, L and R are the same in some languages. And even if a language has both AND pronounces them the same ways as English (not necessarily common), they might not allow an L to follow an R! (Just like how we don't allow R to follow an L)

Oh, and which vowel are we colouring? "i" or the "short I". This is a very rare vowel, following a third dimension (tenseness) that the majority of other vowels don't use. Not common in other languages, either!

So that's the last two sounds.

The first three is a consonant cluster containing another uncommon consonant (w). And even ignoring that, s and k can't always be combined together in other languages.

So literally every sound in the word "squirrel" has something foreign and rare about it to many languages immediately as you start to get past that "s" sound. Brutal.

It's not the trickiest, but it's not exactly easy to say it like the native speakers.