[MRay620]

or more importantly is the fast ring stable or the slow and release builds are stabler?

[mxzhonkey]

Release is the most stable, slow is less, fast is lesser. On some releases this isn't the case, but generally it is.

[Maurizio Troso]

or more importantly is the fast ring stable or the slow and release builds are stabler?

Fast works only on 550, 650, 950, XL (windows 10 born lumias) till further Gabe comunications