Shiek/Zelda in Melee/Brawl is proof-by-exception:
- They were both extreme characters, and diametrically opposed; the type of person who intuitively excelled with one was extremely unlikely to thrive with the other, so extremelyfew people actually played both.
- Some did in Brawl entirely for the novelty (especailly weaker players), but were typically so blatantly superior with one that it will ill-advised. I could probably count on two fingers the players who actually played both justifiably.
- The transform time was both long and random (system-loadtime dependent in Brawl); actually changing mid-battle was just a terrible idea.
- Zelda sucked, plain and simple. Really shoots any theoretical advantage in the foot.
Brawl PT had slightly fewer of these issues (+had stamina as artificial glue), and Ultimate PT exhibits basically none.
Pyra/Mythra have even fewer barriers than Ultimate PT. They are extremely similar archetypes--if Mythra is your #1 character, there's no way Pyra isn't in your top 10 by default. The transform seems very safe/generous. With Ultimate's track record, we can confidently predict that neither of them will be as bad as Melee/Brawl Zelda (or Brawl Ivy).
What I'm most interested in is OoS and kill throws, particularly looking at Pyra. This is part of what makes Ultimate Charizard such a strong team player, often carrying his full third of the team despite the least amount of screen time.
A thought experiment: If Marth/Lucina could down-b into Ike, would you? When?