The primitives of a tpm ("observations", which in practice map to ca-signing keys of various levels of boot stage) are a very poor match for the real world. In order for them to be usefully consistent, you have to have consistent inputs. That means you can't observe the kernel itself, but instead you observe a key that signs a kernel. This is awful when it's Microsoft signing a series of kernels that advance one-way but the TPM doesn't actually ratchet to prevent downgrade attacks (and would be even worse if it did, because then you'd inevitably brick machines again and again). Instead you trust a long-lived CA and God help you when you need to rotate it.

It's even worse if you're self-hosting the signing, to the point that despite a ton of work put into making it safe and understood I wouldn't do it with the current design; a bare-state unlock is more predictable and reliable but I'll make sure to regularly exercise my backup key and header.