I was trying to use their Linguistic Analysis API for POS tagging some text, and even for text as short as 500 characters, the API took over a minute to respond. If I bumped it upto a 1000 characters, the API would just error out and give a 500 status code without any explanation after keeping me hanging for over 5 minutes.
The same was the case with the speech to text API and some other ones as well.
As much as I appreciate there being something like this available for public use, these projects are effectively in alpha state and are not yet fit to be used in serious apps.