Well in theory Darth Tenebrous should be super powerful because he gave Plagueis everything he knew (and Plagueis was super powerful) but Tenebrous has basically nothing to go on and got crushed by some rocks, and when he tried to revive himself he put himself in a endless loop where he lived out his death forever. SOooooooooooo Tenebrous is basically useless.
@wolfrazer: yeah the ST is a mess, really due to them letting Rian do anything. I mean you have ep 7 which was the safest possible movie, they got flak for that, then they let rian do anything resulting in two VERY different movies that are very jarring compared to eachother.
TLJ was so Fked up it's hard to even see feats. For one you have Rey who got literally no training at Ahch-To and somehow shes still better then Kylo who has quite a lot of training, but both of them seem super weak.