I have frequently used flubenol (flubendazole) but have never used any third party agent to dissolve it. I have never experienced any adverse reactions. I use it as a 2 day treatment a week apart if needed. Treat day 1, leave for 24 to 48 hours w/c treat again then normal w/c routine, repeat a week later if needed. I have found it very useful when nematode and/or tapeworms are suspected and the fish are not eating. No adverse affects on the biofiletr and it can be used safely in a planted tank, but it will kill ALL snails so be warned. It is very effective against flukes and other parasites too.
These are my personal experiences of using it, I consider it a real asset, particularly as we can now get it over the counter in the UK.
I do know though that it is a pain to dissolve, I wonder if the various mediums people use to dissolve it in (alcohols etc) are trhe more likely cause of any problems observed. Maybe they afect the chemical characteristics or the way it is used by the fish. I dissolve the powder in pure RO and stir continuously.