To build on the excellent responses you've received already, Fortinet has also recently started labeling their firmware releases as either "Feature" releases (marked "F") or "Maturity" releases (marked "M"). As major releases mature from the first few point releases, i.e. 7.0.1, .2, .3, there are bugs being fixed but at the same time new features as well. As the release becomes more mature, the later releases will be fixing bugs primarily.
Ideally, if you are concerned about stability, stick with the "M" releases (which is 6.4.10 today).
If there are features in 7.0 that you absolutely need, then ideally you have an environment within which you can test it out before rolling to production.