Worst UR, Seriously? She's mid A-S on my book with the right build she's more than what you ask.
She's 4th in damage and has high acc and crit rate due to her high dex 11th currently.
Speaking of damage as long as your opponent ain't a gunner with 10%-15% pierce res. Other char englebert iirc 5% others got none or negative. Means she can dish more damage than the slash meta.
Low agi? No problem at max level she would have 49 agi with the right esper like siren at 100% resonance you'll be having +12 agi. 61 agi and if you have run on snow that 1% adds up to 62 agi that good to very good considering about on the jp average agi on a max level is 55-63.
Not to mentione with siren as her esper. She would gain +15% pierce attack on the esper skill board.
Forget about being a glass canon. She here is a freaking NUKE.
Also an orlandeu slayer.
Speaking from experience. Uses full potential on PVE. Her TP is used as her ammo so that she no longer go near a boss and just spam vacuum slash or nighthawk. You might be adding that she can only buff a number of times. Not if you equip her with a soldier sub job. She retains sharp spear and get hazard form 2 crazy buffs. not sure if it stack tho.
I just pulled her, so my experience is limited. However, all these arguments ignore that the problem is not her agility number, but her relative agility. Sure, you can increase it with espers, equipment, and cards. But the same can be said for all units, and, in particular, Lu'cia with her tmr, element advantage, and range (especially with ranger subclass) makes her a poor choice for arena, IMHO.