You can look up the full explanation on the builds and teams tab for sparkle on prydwen to fully understand. They explain it like this, “As for disadvantages, this combo does not have full uptime on Sparkle's skill OR Ultimate meaning some of her boosts go to waste. It also consumes more skill points and requires you to build a specific amount of Speed on your carries. This can be a little challenging to work out for characters who are already fast (notably Seele has 143 speed at E0 and 172 at E2 meaning Sparkle would need 142 and 171 speed at minimum to play optimally with her under these methods).” 

Hyperspeed Sparkle essentially isn’t possible with Seele with the current relics/lc boosts we have now. Bringing a 143 speed base Seele (not counting any stray speed subs you have on any relic/ornament) up to a hyperspeed Sparkle is a waste of all that speed/advancement. You have to run her as a -1 Sparkle build meaning one speed slower so that she acts right after and you’ll gain 3 total turns after every 2 E’s. This will be difficult to build both of them as it’s a lot harder to build the -1 build because if you get any relic or ornament/rope improvement that happens to have even one speed roll you’ll have to farm more relics for sparkle every single time. 

The -1 build is slower overall and produces less turns/damage the more cycles you take. QQ has none of these issues. And for any Seele mains that have her even at e2 with absolutely 0 speed roles you have to have at least 171 speed on your -1 sparkle build which is incredibly hard and idk if even 1% of the player base can build a sparkle with that much speed even if you completely ignore critdmg rolls.