I'll start with Turborepo. Turbo is primarily a task runner for `package.json` scripts with some caching... and that's basically it. If that's all you need, then great, but if you're looking for more functionality, that's where moon comes in. moon is more than just a task runner, we're aiming to be a repository management tool as a whole. This includes project/code ownership, direct CI support, future CD support, code generation, hooks management, constraints, release workflows, and much more. With that being said, we do have a comparison article against Turbo: https://moonrepo.dev/docs/comparison#turborepo
As for Nx, they're more of a competitor than Turborepo. Nx and moon are aiming to solve the same problems, but go about it in different ways. Nx is Node.js based and requires heavy adoption of their ecosystem (@nrwl packages) and their executors pattern. In the long run, this becomes a heavy source of tech debt, as your dependencies are now tightly coupled to their packages and release timelines. With moon, we wanted to avoid this all together. There are no coupled dependencies, and tasks are ran as if you ran them yourself on the command line. No abstraction layer necessary. We also want to embrace a language's ecosystem as much as possible, so moon adoption should be rather simple and transparent (at most each project has a moon.yml file).
But to your last point, we agree, multi-language support is a massive advantage. Having both backend and frontend code in the same repository, powered by the same build system, is a massive win in maintenance costs and developer time saved.