In addition Czech has the sound ř, which is almost impossible to pronounce unless you learn it in childhood. I don't know of any other language with that sound. Tongue-twisters will almost by definition explore the weird sound combinations in any language, but Czech is a fruitful hunting ground for these!

Interestingly, the geographically close but linguistically unrelated Hungarian makes up for all the vowels that Czech eschews. They're almost polar opposites in vowel usage.