HHVM

Primarily focused on Hack, making it less versatile for projects that need to run PHP or other languages without additional configuration or potential compatibility issues.

Setting up HHVM with traditional web servers like Apache or nginx requires FastCGI configuration and package management, which can be more involved than using the built-in Proxygen, as noted in the installation steps.

The Hack language and HHVM have a smaller community compared to mainstream alternatives, potentially limiting third-party libraries, support resources, and adoption outside Facebook's ecosystem.

The just-in-time compilation introduces initial latency and memory overhead, which might not be suitable for applications with frequent restarts or in resource-constrained environments, a common trade-off with JIT systems.