Kung Jin's pretty simple. Really easy to convert into the same combo off of almost anything, and not hard to keep good spacing due to his great range. I've heard a lot of players say he's a great place for beginners to start. Ancestral seems a little more complicated to play effectively though.
I think if it weren't for the run combos, Cassie wouldn't be giving people so much trouble. Having to worry about the run can really mess with the execution since it's constantly on your mind. The good thing is most of them are all universal combos, at least the midscreen ones. They just have different enders, so it's not hard switching b/w variations. You might get frustrated trying to pull the run combos off online though. I know players who use "dumbed-down" versions just for online play to keep consistent damage.
Also, it's tempting to play her super unsafe and just throw out b12 getaway flip like it's whatever since it can seem tough for people to punish and there's really no other starter with that much range, but once your opponent figures it out, you'll get wrecked. I think the biggest thing about being a good Cassie player is learning to play safe (b212d+1+2 is a great hit confirmable starter), and then you'll be pretty solid. She's an amazing character that is definitely worth learning, so if your only misgivings are the learning curve, stick with her!