Though both share the ability so securely execute multi-tenant workloads, but they make very different tradeoffs when it comes to compatibility vs performance. To compare:

Firecracker is great if you want to securely execute an OS image. It has the benefit of compatibility with many existing programs, but that comes at the cost of some overhead.

Hyperlight is great if you want to securely execute program runtimes. This requires bespoke guest bindings, but it has the benefit of having less overhead.

There’s a place for both approaches, and I see both happily co-exist.

From what I understand Hyperlight's boot process is more similar to a microcontroller than a PC (although it use your CPU architecture) - the VM directly boot into your code. Unlike Firecracker, Hyperlight VM doesn't have any hardware while Firecracker do have VirtIO devices, serial console and keyboard so that traditional operating systems can be adapted to Firecracker. Host-Guest communication is done with shared memory.