I don't have Kazuha on my main account and have been using Sucrose C6 to benefit from the elemental boost she gives to other team members. If we are talking about an elemental damage bonus, then Kazuha will outshine Sucrose in the case and offer multiple elemental damage boosts to your team members. He can swirl multiple elements, and their elemental damage bonuses will have separate counters.  On the other hand, Sucrose's kit focuses on buffing the overall team's elemental mastery, which can help you amplify reactions to deal a massive amount of damage. You can see one-shot enemies with vaporizing or melt reactions after buffing up your team's elemental mastery with Sucrose. Conclusively, Sucrose and Kazuha are both different characters with unique abilities and have their team-specific requirements.