TL;DR: My (shitty) evaluation is Jack of All Trades, But Still Pretty Good. Merlin maximizes her effectiveness.
Quetz plays the hybrid-role of a Critical Buster Servant with supportive abilities in her targettable Buster buff (doubling as a targetable Guts) and a higher Charisma rating than Ozy. Her one-turn-lasting 3rd Skill, which is jam packed with her Critical Damage and Absorb skills (and Riders already have the highest Star Absorption rate across all available Servants on NA), allows for some nasty damage, on top of also being a 20% battery that scales to 30%
Rider Kintoki may compete for her position as a boss killer, since Kintoki also features a battery but charges to 50%. Considering you got all copies during Rashomon, his NP Damage likely outscales Quetz's Level 1 NP. Both of their Card Boosters feature the same minimum and maximum Damage Boosts.
On the other hand, you may prefer Ozy for a supporter due to his team-wide battery and being the only Servant to boost Skill Buff Chance Up (plus his Charisma), while also being a good damage dealer due to Imp-Privilege.
It kind of boils down to team composition. Is there a boss with a huge HP pool, and you can field really nice Star-gen? Are you gonna roll for Merlin and run double Garden + Hero Creation (even just one is good enough)? If you like to see huge yellow numbers, Quetz is a really good, if not the best, choice.