I tamed one of these things, and use it pretty commonly to take down therizonasaurus. The spiked tail adds a knock back to the theri making it difficult for the theri to do much damage. Only downside is the movement is so slow and stamina is pretty much non-existent.
With the new TLC the stego can easily become a must-tame early game dino for many players. It has 3 stances when ridden you can change between pressing C (crouch):
Heavy plate, boosting thatch gather rates and applying a heavy slow on hit targets.
Sharp plate, boosting berry gathering and attacks penetrate armour (useful for PvP).
Hardened plate, boosting wood gather and halves damage taken.
For combat, heavy plate is great for effectively rooting enemy and wild dinos in place, whilst hardened plate is to tank everything that comes at you. Sharpened plate shines together with allies that can slow and/or tank whilst you focus on dealing good dmg.
The alternate attack of the stego can impale creatures up to the size of roughly a parasaur, dealing damage based on attack damage. In order to do so the target must have a certain amount of health left, which is highlighted with the crosshair turning red when looking at the target (unsure how it is shown without crosshairs).
These big guys are not to be underestimated. They have a slow speed, but don't let this fool you. Once tamed, they can gather insane amounts of wood, berries, and thatch. I suggest taming these as soon as possible. Use upgrade points on health, melee dmg, movement speed, and stamina. Hope this helped.
The stegosaurus is great for wood gathering if you need a bunch and don’t have a lot of time because of their massive weight capacity. Even though it’s a pretty slow tame it is great for gathering resources in abundance.