Is Manuscripts.io a legit tool? Any experience with it?
A good rule of thumb is that when you get something for free on the internet, you're the product, not the user.  There is often something being done with some or all of your user information or content you provide to generate some cash flow somewhere.  Or, you may be an advertising target.

An exception might be some academically developed mechanism supported by some society.

Poking around the website you point to, though, I see no Terms of Service immediately available, and this is a red flag.

There isn't necessarily anything inherently wrong with an arrangement like this.  You might find the service provided worth any use of your content/data or putting up with ads.  SE network, in fact, does this openly, and we're still here.