After thinking about it again I see how Ferocity can be interpreted as giving the units an ability that is applied only once. However, I think Ferocity is an unique case so Stampede and Elite Training don’t necessarily have to behave in the same way as Ferocity:
Stampede only lasts until the end of turn, so not sure when you would lose control of a unit during your turn?
Elite Training is different in that it specifically makes you choose two units (and also targets them).
Related Question: If I play Ferocity and then play another unit from hand does that unit gain the bonus from Ferocity?
Stampede only lasting “your turn” makes it a different case as I don’t think there’s a way for you to lose control of it on your turn (is there?)
Elite Training is a good example though, and what makes me think it “sticks with” the unit. You’ve essentially “imbued” that unit with new properties until your next upkeep, and nothing suggests it can lose those properties other than when next upkeep rolls around.
Made Ferocity an interesting “anti-kidnapping” tool of sorts: it broke down my ability to trade two for one on the heroes!