While I don't think a better character somehow discounts the usage of an inferior character, since Lapis is just worse by nearly every definition except being a potential user of Levin Sword that Kagetsu can't use quite as well, I think the most important thing here is actually how inflexible she is. I think realistically, her only use is being a slightly better dodge tank in a Corrin fog for maps that don't have a 1 square wide key chokepoint. Being a Mage Knight doesn't do enough to supplement her weak offense, her speed is hampered by her poor build.
But most importantly, the game doesn't ever really need more than 1 sword focused character mostly because Alear and their dragon-ness tends to break most emblem rings, and Kagetsu, albeit better fit as an axe user, can also fill that role. I think really the only one that ever needs 2 sword users is the palace map (Ch 14?) where there is a whole side of axe users that 2 sword users basically duo the entire area and still reach the thief in time. Kagetsu can basically fill that space, Alear the other. But aside from that, swords aren't honestly all that great. Fogado can fill as a pseudo Levin Sword user, as he will always have that magical weapon niche that the classical bow user that hasn't reclassed from a mage can possibly fill. Radiant bow is pretty broken so his smattering of magic ends up being bigger than the numbers suggest
She can be a hero to be a double chase attack bot with Levin Sword, and she will occasionally contribute to an armor unit kill at the most. Her high avo can probably help maintaining her at full HP, which makes her an in general very good avo tank along with her passive as she can be placed in the middle of a lot of enemies and contribute to double chase attacks.
Her hit rate buff can also contribute to her becoming an axe user, which can supplement her generally low attack, but I don't think her speed growth/base can even make up for how much speed she will lose using an axe. There will also just be better axe users in general.
On a side note about Yunaka, I think because daggers upgrade so well, it makes her crit rate buff pretty valuable. Giving her Byleth's luck buff and her frequent crit and above avg speed growths along with Corrin makes her a very good frontliner. Peshkatz gets up to 21 might 30 crit, Yunaka gets 15 crit from passive, Bylut gives 12 luck, that's before counting engraves. She dodges better and doubles more than Zelkov and crits basically 15% more often, which 3x dmg * 15% is that she does on avg 1.3x more dmg than Zelkov. There are a lot of flaws to this math esp considering def is a flat decrease b4 the crit, but there are even more factors to consider like Yunaka having better speed
Also it's probably not feasible to get max Pesh and max bond with Byleth in time for this to even matter, but it's the type of growth that Yunaka can have, on top of just more chapters to work with where she can be funneled with early stat boosters, which is a whole other subject.