I'll second the FL Studio Vocoder. I've used the FL Studio vocoder since before it became VOCODEX and I have to say that it is the only vocoder with wich I have been able to achieve consistent intelligible results. I've also found it possible (using the right synth) to achieve the vintage 1980s "Freestyle Don't Stop The Rock" type of robot voice with it. As far as any Vocoder and becoming harsh I'll say this:
A vocoder is a raw tool capable of splitting up a "modulation" signal (a voice) into a discrete number of frequency bands and then modulating a carrier (synthesizer) within those discrete frequency bands. At anytime it is possible to have a little "too much" energy in any one of those bands causing an unpleasant "honk" at a certain frequency. Afterall, with that many bands the Q on each filter is probably quite high so: high resonance = high probability for a "honk" of energy within each band-pass filter.
Bottom line: Apply a limiter, compressor, or a multi-band compressor after your vocoder and all should be able to be kept "under control."
I used the FL Studio (i believe circa version 7 or 8) in this "so old it's completely irrelevant" mash/mix (the "DJ Giant G tag at the beginning" and the Timbaland vocal that I actually re-spoke/rapped into the vocoder myself at about 2:09:
ahhh i cam help here,, 2 really good, and really simple to get on with vocoders i use, are,,orange vocoder by prosonic,, and 1 ive been using quite a bit lately,is "the mouth", u will need reactor 5 fx to run it tho, but i dont think u will be disapointed in the results,, hundreds of presets also, so if 1 aint quite what ya want, and u dont wannamess about with settings ( although easy to use also ),