It only matters that the client is open source (note I didn't say free software). This is so you can read the code and be proactive about issues in it. For example the mixpanel client for Android caused one thread to be created whose sole purpose was to fire a timer event every 60 seconds, which was used in a second thread to see if there was any work to do. It is a lot easier to figure out stuff like that is happening from the source than to try and work out who is responsible for extra threads showing up in the debugger. If there is a suitable license then you can at least modify and redistribute the changed code.

We do actually do our import and custom processing in MongoDB, so that side is covered.