Honestly, as someone who considers King Sombra as best pony of the whole franchise and has developed an idea of who and how he is, I really wasn't disappointed in the least. In fact; I quite loved what we were presented with!
Though, if I have one critique, Sombra didn't finish the job when he should've. He destroyed the tree and had the Mane 6 where he wanted them...but just left them. For being the ultimate planner and great tactician he is; that was a HUGE blunder. Granted, he had a backup plan in messing with their minds in invading Ponyville and mentally enslaving the citizens of Ponyville and Canterlot had their desired effect in deflating them further (really; only Discord saved them there). But still; blunder is still a blunder.
That aside; I was perfectly happy with how he was portrayed. While he never said much and was completely goal-driven in "The Crystal Empire," I can accept his more ego-driven personality in the S9 premiere. I mean, we get small glimpses of that in S3 and S5, but it's enough for me to buy it here. Plus, even with that personality, he is still goal-driven. He invaded the Crystal Empire like no one's business and actually succeeded! Had he not been an apparition in S3; I doubt Cadance would've stood much of a chance. Likewise; Canterlot stood no chance, especially with the royal guards not wanting to attack the Ponyville citizens; GENIUS move on Sombra's part there.
Not to mention; I LOVED how Sombra was not impressed at all with Grogar. I mean, Chrysalis and Tirek were definitely intimidated, but not Sombra. Everytime I see that scene; I just have this satisfied smile on my face.
So, in the end, I was perfectly happy with Sombra in season 9. He was still at his tactical & strategic best, was only defeated due to Discord's interference, and left enough of an impression on everyone that even Canterlot's defenses needed to be reworked. Yeah, he blundered in not finishing off the Mane 6 when he shouldn't have, but he had backup plans in place to help mitigate that. In the end, I still love my Shadow King. <3