Is Manuscripts.io a legit tool? Any experience with it? A colleague just tipped me about manuscripts.io which advertises itself as a collaborative writing tool, with some really sweet features like importing docx and LaTeX; searching and adding citations directly from within the platform.. It is also free! 

That being the case my metaphorical spider-sense tingle a bit, if it sounds too good to be true, it probably is. I am wondering if anyone knows more about this platform. Are they secure, is the text submitted used for some other purpose (text-mining or so) in the background? In the cut-throat competitive nature of academia, it does not surprise me if it was a scheme of sorts (yes, I do realize how paranoid that sounds). 

In any case, just wanted to know if anyone has looked into it and if so, what came out of 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.