I'll have to re-check; I've been able to read several longer fics without problems, but I know of a Sony bug that could clash with them.

Sony's epub reader can't deal with segments larger than 300kb; if the conversion isn't breaking up the HTML pieces into small enough parts, they'll freeze when it tries to read them. (Epubs are just repackaged zip files of HTML + some metadata. If you change the extension to .zip and unzip it, you can see the raw contents.)