I know Yuna is a popular FF character, but her unit in FFBE isn't really all that great. To break it down:
As a healer, she can't dualcast, and while that's an easy problem to fix, it means your magic tank isn't using the best magic tanking weapon (Holy Rod), or you're "wasting" a materia/accessory slot to give her dualcast. She can only cast Curaga, and even dualcasted that means she can only heal something like 6200 health per round. Usually that's enough, but sometimes it isn't. If someone dies, she can only normal raise them (at 30% health instead of full-raise). She can cast re-raise, which is about the only thing that she has going for her as a healer over a 3* healer like Roselia.
As for her summoning damage, she does less summoning damage than enhanced Garnet, and her gimmicky "use the esper gauge to cast spells" stuff is weaker than just using her to summon Bahamut with that same esper gauge. It's true that 3* espers can deal some nice damage if properly capped with a summoner, but like I mentioned, if that's your goal, just use enhanced Garnet for better results.
As for Lulu, she was power creeped before her banner even came out in JP, and we're getting this banner like 3 months late, so yeah. Lulu is basically a 4* mage hiding in a rainbow designed to troll players pulling for mog synergy.
Far down the road, Yuna's 7* form thankfully does fix a lot of her problems. She becomes a respectable healer with a much better toolkit, but she's still inferior to the real healers like Fina and Ayaka. She's also a worse summoner when compared to 7* Eiko or Citra (unarmored Light Veritas, future unit).
Lulu's 7* form... gets tornado so she can chain in 10 mans, but she's still pretty terrible,
I hate to shit all over the banner because I really liked FFX, but those are the unfortunate facts