The book I downloaded from MobilRead was the epub version of 'Kings, Queens, and Pawns' by Mary R. Rinehart as edited by Crutledge. Perhaps the problem is with my computer but it does not generally crash with other programs, but it does consistently when I try a search (for 'chapter' or 'england' or 'german'). The system either crashes immediately or nothing happens for at least a few minutes after the first find and I click the find next button. After which time I gave up the search in book view. The word count seems to work fine. I downloaded the book again, tried it with sigil search and had problems although sometimes the search seemed to sort of work, but other times for the new download it crashed or nothing happened on find next.
I tried a 3rd book, The Richest Man in Babylon.epub edited by =x= on MobilRead. For this one, the book view search seemed to work perfectly.