Well, I've just noticed that I can't really confirm my original described problem.
I could see many times that after another reboot RAM disk file still has an old modification date, and I always thought it means that last save attempt failed.
However recently I've checked the actual content of the RAM disk and found out that it still contains the files from the last day, meaniing their creating/modification date were later than modification date of the RAM disk itself. Due to the nature of the content inside (I'm using RAM disk as a folder for browser and its cache) I can't always say what data and how much of it has been lost, because it always gets regenerated quite fast (feeds are being updated with all lost content automatically, logged-in statuses/cookies are quickly fixed by logging in with saved passwords etc). It also can be affected by browser crashes.
I'll look more closely next time something like this happens to better identify the issue, and report back if I find it was really the case of RAM disk save failure.