After a freaking hour plus, I have determined that ArcGIS Desktop 10.6.1 "Features To JSON" with the GEOJSON output option ONLY WORKS with SHAPEFILE INPUT.
Come on, ESRI....
"Features To JSON" repeatedly crashed, after output for the first few polygons, on the 4th or 5th polygon geometry, WHEN USING a FILE GEODATABASE OR ARCSDE polygon FEATURECLASS INPUT.
I tested with 3 or 4 different polygon featureclasses that were in WGS84, whether or not within a featuredataset, in FGDB or SDE, no matter, it crashed every time with a useless "99999" err msg. The log file was also useless. Clearly it was choking when reading geometry.
Copy the same featureclasses into shapefiles, and the GEOJSON output option works fine.
I've struggled with this bug since last summer, finally found a workaround, and I'm flabbergasted.
ESRI, I can't believe your code doesn't support FGDB or SDE, only supports legacy shapefiles as input.