I think this skipped out on the downside of using Firecracker: the host underneath needs to be either Baremetal(AWS) or VM with nested virt support(GCP). This creates additional complexity to manage such a setup in production. Moreover, since both nested-virt VM / baremetal both comes at a very high spec by default, the economic would only make sense if you are at a scale that could saturate(or over-saturated like the article hinted at) these resources.

Very interesting blog post nevertheless. Looking forward to read more!