ACMs roots are cross-signed by Starfield (GoDaddy iirc), while they wait for their inclusion in the browser roots... so I can't see a difference with LetsEncrypt? AFAIK either you have the processes/tech in place to secure your CA and issue certificates, or you don't.

https://mozillacaprogram.secure.force.com/CA/PendingCACertif... shows Mozilla's in-progress certifications -- including LetsEncrypt, Amazon, DocuSign, VISA, a bunch of governments, telcos, existing CAs, and others I don't recognise. Cross-signing is a pragmatic solution for older client devices (eg. abandonware Android phones) for _any_ CA root, new or otherwise.