Define good. Because frankly I see a subpar champion with:
1. Low AP ratios on top of already low base damage (140 DAMAGE on a rank 5 Q that forces you to HIT someone? On a CD similar to Sion's auto target stun? Like what the ****)
2. Medium length CD's, even with max CDR.
3. Her leap in isn't very accurate/effective. You are forced to leap into the last person hit, you cannot choose who to leap to. So that person who is about to do massive damage to your team, who just happened to get hit before another guy gets hit, now walks away freely. It's also awkward to throw your sword, watch it HIT someone, and watch them CC you before the animation is over and have the teleport fail.
What are her GOOD traits?
1. She is naturally tanky. Both from shield, and health/ar stats. This is a good aspect for someone going to tank.
2. Her mana costs (Besides the ult, which I mentioned a few pages ago is a Bull**** amount of mana) are quite fair for what the abilities do. Particularly the shield not rising in mana cost (although not lowering in CD by rank either)
3. She has fairly solid laning. She's not forced to go into the jungle to be max effective, like a Mumu or Rammus.
With that said, I can't call her a good champ. I mean, she's not old Yorick bad, but she is frankly subpar compared to other tanking options. There's no point in making her your main tank, and why play her as an off tank/tanky DPS if she cannot effectively go that route because of her damage/noticeably poor scaling?
As for the comparisons to Mumu and Sion, I think some of them are pretty fair. Sion's shield is VERY similar to her shield ability, but tends to be better. 1-1 scaling ratio, shields him from damage, and he gets to choose when to explode the shield after charge up. She gets to shield and it HAS to explode in 3 seconds. Not a lot of variety on waiting for the right moment to blow up. Her Q is a stun just a tad shorter duration than Sion stun. His rank 2 stun does more damage than her rank 5 stun AND has better scaling AND he can target from a range. Her ult is a better Brand Fire Pillar, although he can end up doing more damage with the passive health ticks sometimes and the damage boost from the target being already on fire.
I mean maybe you think it's unfair for me to compare her to champs, but if she's not comparable to "good" champs, then how can you call her good? "Oh you can;t judge her next to Sion/Mumu" "Well that's cause she sucks and it's hard to justify using her over characters like that or characters worse than that but still comparable to them".
Bottom tier 3, tier 4ish character. Anything higher and frankly you're lying or there's some bug out there that lets her be more useful. That's being generous as well IMO for her to be in tier 3. That would be the absolute most credit you could give her, to be the bot of that.