After 4 episodes of Shannara, I didn't think it was bad either. I just didn't think it was awesome like Game of Thrones or Vikings or even the new Beowulf series.
I get the feeling Shannara was targeted at younger audiences like 14-25 YRs. It seems like the younger characters just don't act very seriously enough. Maybe they just can't act. Or just bad writing.
I mean if my world were potentially nearing armageddon, I wouldn't be cracking too many jokes like some of these characters.
I've only seen 2 episodes of The Expanse. The characters here definitely act very seriously. I like that. They should be serious. Their worlds are dire. But so far, it hasn't hooked me like Battlestar Galactica or especially Game of Thrones did. Hopefully I'll be hooked after episode 3.