The stack for the VT requires a heap allocation [0], which ok, not huge deal for most scenarios, but something to consider. Reactive programming will avoid that. For example, for a service that doesn't do much IO (like an in memory pubsub thing or CDN) you would still want to use reactive programming if you care about performance, since likely the code will be simple anyway.

[0] https://openjdk.org/jeps/444