I suppose the only thing is that you wouldn’t be able to upload an image to the instance as part of a post - you’d have to upload it somewhere else first, to then be able to refer to it.
For the detractors, register a throwaway account at some random instance, and use that if you want to test it out.
If you’re able to properly pore through the source to check it’s not stealing anything, then you’re capable of scheduling your own posts. The Lemmy API is very simple, it’s not rocket science.