Are there Slavic words with separately pronounced "s" and "z"? It's quite a tongue-twister.
As for "X" it exists in some Slavic alphabets, for example in Slovak (but apparently only in loan words). In Polish it theoretically doesn't, but actually does (loan words use x when the word is new, and with time it's replaced with ks).
Mst and vzt aren't hard to pronounce? Pomst and wzwód are regular word.
As for s+z - interesting, in Polish there's no s+z pronounced separately. "Szadi" would be "z zadu" and you do a pause between the z letters. And if we have "from $PLACE" where place starts with S we change "z" to "ze" for exactly this reason. So "Z Rosji" and "Z Zimbabwe" but "Ze Szwecji".
In Russian, the combo s+z is also effectively pronounced as a geminated z. But it's still spelled "sz", because Russian orthography tends to be morphemic rather than phonetic. So from the perspective of reading familiarity, using "z" as a digraph marker would cause a lot of confusion on how to read it properly.
"x" at least has the advantage that it's foreign to almost everyone, and thus forces you to look up what the hell it's supposed to mean! But in general, I think that diacritics work much better for readability of Latin-based Slavic alphabets, with Czech being a decent starting point ("ch" aside).
As for the consonant clusters, they are easy to pronounce to Slavs, because we're used to them. Ask a native English speaker sometime what they think about a combo like "vzt" :)
(The reason why we ended up with these unusual consonant clusters is because of the loss of yers. For example, "pomst" and "мстить" are derived from proto-Slavic "*mьstь", where "ь" was originally short "e". This is also why vowels show up in the middle of those consonant clusters sometimes when words change such that the syllable that used to have a yer in it becomes stressed - as in Russian "месть".)
As for "X" it exists in some Slavic alphabets, for example in Slovak (but apparently only in loan words). In Polish it theoretically doesn't, but actually does (loan words use x when the word is new, and with time it's replaced with ks).