Hard work writting this
those things are hard for Polish people : pięćdziesiąt, sześćdziesiąt, dziewięćdziesiąt, as those are rare words with "silent letter"-ć
I don't agree, Polish people don't have any problems with it. I think it can be problematic to people who learn Polish as a foreign language, but for Polish people these words are easy in writing and speaking.
Well, I have to stop and double or triple check every time I need to write one of those three. But I do not do it often, I guess, if you have to write those words more often it's easier.
Unless they haven't practiced much reading and writing much. I haven't seen a case of full reacquired analphabetism, but some knowledge might still decay.
These numerals are a curious case, where the spellings follows the logic closer than the actual pronunciation. You just take the basic numeral and stick „dziesiąt” at the end.