Let's get straight to the point. Kentrosaurus is going to be a cut above. It's piercing gaze alone will be enough to send even the largest carnivores running. It will slash the competition. Tribes would be foolish not to stick this in their arsenal. It may be a hard tame, but I'll take a stab at it. Did I mention that it's pointy?
Where would you even put the saddle? The only “clear” spot I see is the neck or head, and the poor guy could not even hold your weight on its neck or head. Then again, you could have an elevated saddle over the spikes.. that would be impractical. Too small to carry you, but great for tribe-angering.