Quarkus has the following drawbacks:
Quarkus is less flexible compared to spring, or in other words, it is more biased and coupled to specific implementations. However, the implementations work and you have less things to choose and worry about. However, in case you want to integrate a specific or custom library, you may hit limitations or lose support for native-images, especially when that library is based on reflection. Therefore, check your requirements and technology stack early on when making your choice.
Since quarkus was born in 2019, it is modern but also less established. It will be easier to get developers for spring, but we already consider quarkus mature and established enough for building production-ready apps.