Are the limitations of the Download() function documented somewhere (akin to the whitelist provided for libraries)?

I'm trying to fetch data from an external source using the QCAlgorithm.Download() function. 
The data is part of the universe selection, and hence scheduled daily.
When running backtests, all goes well until 100 downloads have been executed. 
From then on, all calls to Download(...) return an empty string, without any logged errors/warnings/exceptions.

I've looked at the algo reference documentation which just states:
"You may want to import a single static reference file - such as loading a daily symbol list, or an AI-training file. This is possible with the Download()"