There is a difference between accidential mistake that was overlooked and consistent errors that come from the lack of skill.
Just because someone makes mistakes here and there it doesn't mean they suddenly lack the right of pointing mistakes in others work, especially when they see imperfections in their own. You can't just come up and say "You are all wrong", you need evidence. There are numerous evidences of T-N mistakes and errors. Even so, GUIS subtitles are tolerable and can be easily fixed. T-N's work, mostly their early work though, has some inexcusable mishaps that affect the enjoyment viewers can get out of the series. If only small part of apple began to rot you just cut that part out and eat the apple, but if half the apple is rotten you'd probably just throw it away. T-N before 2009 are rotten apples.
Another important point one must consider about the "scrubbing" movement is that T-N is still technologically stuck in 2005. They still do subtitles hardcoded into the video, out of the fear somebody could steal their precious work perhaps or out of catering to technologically impaired part of audience, doesn't matter. The difference between softsubs and hardsubs is that former can be easily edited by anyone. Say you encounter an erroneous line in Over-Time subs, you just edit the script file and call it a day. But you can't edit T-N subs for yourself, or for anybody else's gain, unless you go through tedious process of making hardsubs into softsubs. This is part of the reason T-N mistakes are so annoying, you just can't get rid of them that easily.
You must consider that most of the scubbers hold at least some respect to TV-Nihon's work and do it not just out of spite towards the group, but out of good will to improve already existing translations for everybody's enjoyment.