I also see crashing of Unity 7 very, very rarely and I have my PC running for dozen hours. I'm currently using Trusty (development branch) of Ubuntu which I upgraded to from 13.10 in the first week of 2014 and Unity 7 crashed only once (in this period of time).

I have a bad habit of opening websites and not closing them so I end up with about 90 tabs open in Chrome all the time this results in very high RAM usage. Usually all my RAM is used and then about 1 GB swap. This doesn't bother Unity 7.

As for your points:

1 - you got used using each window in full screen mode, la windows

No the only windows which I have maximized are that of web browsers and LibreOffice all the rest is at normal size.

2 - you are turning off you computer regularly, so unity is refreshed

I'm turning it off after a day of use.

3 - you do not use libreoffice for real productivity work

I'm using LibreOffice very rarely so I don't know how it behaves but seriously I don't believe that it crashes the whole Unity. Maybe you see the apport window the one that is used for reporting bugs.