Me and my friend are using STS engines for our buggies.
Even the D21E are quite stable even when bashing up to 170*C (so far, touch wood).
For $8xx these STSs can be a really good bargain.
Ran the engine with 16, 20 and 30% nitro,
hot, medium and cold plugs,
and they all seemed to run very stable.
Only a minor leak in the backplate,
simple fix with a backplate gasket.
I bought mine was a rotostart backplate for $880.
I didn't shop around for it though,
just bought it from DinBall
Not sure where you can buy with best price,
since there are not too many shops carrying STS engines.
But I'd say they're quite reliable for weekend bashers like me