I made a simple perl script for checking how my indexers perform. The output is extremely basic, but if you would like to try it it’s available here: http://dpaste.com/31K3NJJ.txt
Select the text, paste it into an editor, set the API key (and host/port if they are non standard), save it as sonarrstats.pl and run. It will create the file sonarrstats.html in the same directory. You can run it on Windows with no extra fuzz if you install Activestate Perl. On Debian you may have to install libwww-perl and libjson-perl.
It generates some numbers for total downloads and failed nzbs from each indexer, by each resolution and each series.
I know the code is ugly. I’m kind of hoping that will provoke someone into writing something better.
Edit 2019-05-18: Added failed nzb statistics for series and a single table for all series at the bottom.