What looks to be the problem is that her skill is the only source of taunt. A single dispel on that and she seems useless. While the team has plenty of dispels to counter ability block and stun (buff immunity/shock isn't a worry as she cleanses herself)
Shore didn't have this issue because if you dispelled his taunt, the cooldown of the skill went down
GK doesn't have this problem because his taunt triggers based off incoming crits, which are almost always there
Old Ben regains his taunt, even after it being dispelled
SiT taunts if a Sith breaths or gets breathed on
Sion gets taunt based off of a debuff that he spreads and keeps spreading
I could name some other tanks that aren't really used in arena that can have much more than one taunt (even a 4 turn taunt) that never really sparked in arena. It's not really about taunt length, but how often the taunt pops up. And all the other cool things tanks bring like group hugs or dispelling the enemy team and what not
To me, it seems like she's a high damage tank, just as GMY is a high damage support. Juhani seems extremely durable, and we will see how hard she hits. Seems like a Fulcrum 2.0 if you ask me. Good kit, just doesn't fit in with the demands of arena