Cyrillic has custom characters for character pairs in western European languages. For instance, {a,e,i,o,u} are in both Cyrillic and English (for example), but Cyrillic has custom single characters for what English would spell as {ya,ye,yi,yo,yu}, and then similarly for what English writes as {sh,zh,ch,shch}.

Cyrillic also has a couple soft vs hard annotation single characters, which I'm not able to explain well in terms of their cousin western European but still Indo-European languages.

So you get horrendous spellings if using Latin characters for the Slavic sounds, like you see in Polish.

(Disclaimer: i'm a native English speaker who had years of western european linguistics, also took Russian and had a Polish grandfather).