This reminds me of the Zophar argument.
In that there was a question of "Chris lost to Zophar, who the hell beats him when his supposed slayers don't?"
I remember having to explain that those who claimed Chris = Zophar Slayer were jumping to conclusions of "Magic Immunity trick faster than Fate Storm = Win!" when that wasn't the case, as Chris showed. Then went to show the number of characters who actually have, at very least, some argument for beating Zophar, and the list wasn't large, but there were still a number of them.
Yuna's in the same boat really. As Elfboy noted, there are a bunch of people who just flat out destroy Yuna (like Tir, who many agree is faster, and Yuna has nothing in the way of equipment to guard vs. perfect Instant Death), and people who at least have some argument for beating her (Kefka, for example, depends on how you respect his speed and her evasion; the fact that he actually beat Yuna when they fought is proof that there's enough people willing to view things in his favor, so the interp isn't that farfetched.)
Yuna's really good; one of the reasons people don't like her is cause she's really good based off a boring strategy of "Go First -> Win!" which doesn't really have any flaws to it, and she's fast to add insult to injury, so her matches are basically "does she get a turn? If yes, win! If not, no."
Jenna though? Lets take a look at what happens with a form chaining:
A. Her first form is now pretty much just a roadblock. The HP of each shift is pathetic, and you can't do more damage to just blast through this phase. This means she suddenly absorbs 4 hits from someone.
B. However, said "roadblock" also has 2 shots of absolutely killer damage. Even the most durable of bosses will be taking a hit from Bahairava, let alone 2 will kill most (people like Fou-lu and Myria1 who absolutely scoff at physicals are the only ones I can think of offhand who shrug the attack off.)
C. This form also has Debilitate; chalk up the ability to kill all stats between Baharaiva's (at least, from what I understand, she can use this in the form that doesn't have Baharaiva; I never saw either attack)
D. Once you get through that first form? Now she's got this form that's immune to the big 3 elements, alongside Earth and Wind (not uncommon Elements, keep in mind), and physical immunity to boot, that can is actually durable, and can rack up damage quickly.
There's...I dunno. How many duelers can actually deal with all that. I mean, yeah, Jade beat Jenna...assuming she had one form. The form shifting thing, however? Now suddenly he has to deal with that same Form 2 with 2 Baharaiva's lowering his HP dramatically, which isn't pretty.
And Jade's not what we call a Low Godlike either.
The point is...Jenna becomes ridiculously good to the point of near unbeatable in the DL; you have to survive 2 Baharaiva's, AND then manage to beat her final form's defensive game, with the issue of probably taking a hit to your stats (how big of one, not sure, but its still insult to injury.)
You'll notice there's a big difference between Form Chained Jenna and Yuna suddenly; the number of duelers Jenna Form Chained can't beat is far less than Yuna's.
In fact, the sole reason we DIDN'T rank Blue from SaGa Frontier? Cause he is too good. There's no reason to not allow that Overdrive Loop since he has full claim to it in game. This Loop ends up having him do...what? 80 Mega Wind Blasts before running out of MP? That's 240k SaGa Frontier damage, NOT SOMETHING ANYONE CAN REALISTICALLY SURVIVE. What about Magic Immunes? Still not winning it; Blue can squeeze out enough Physical damage from Light Sword or just equipping Guns (I see no problem with letting Blue equip a Gun, just not allowing him techs or stats from it; its akin to allowing Gordon Axes in the DL for a damage boost on his uniques but not Axe Techs, as far as I'm concerned.) What about PCs who are faster? Don't exist, barring Time Lord, since Overdrive has initiative (hence why the argument of "You must survive Overdrive to win" comes up with Timelord before anything else does; granted, Timelord's damage < Blue's Damage, let alone he also lacks Stasis Rune, hence why TImelord can be beaten)
yeah, there's a clear line between "High Godlike with small number of spoilers" and "Godlike that can't realistically lose ever." Yuna's clearly the former, Jenna Form Chained is the latter.