In one of youtubes videos about scalling web sites I found the statement that, however, in term of scallability, noSql is easier and more flexible, though it is better to use SQL for data important enough to not want them be lost (not necessarly secure). Why is so? What mechanism make SQL safer in such term? I'm very VERY beginner with databases so easy answers will be appreciated.
I guess what your sources are refering to is ACID compliance. There are lots of SQL databases which provide ACID. Most NoSQL databases, however, do not provide this for the sake of better scalability. Here is an article on Quora which might give you an explanation.
For both categories of databases, however, if one wants data not to get lost, regular backups are important, and that has nothing to do with "SQL" or "NoSql".