Yes it depends on your site as well, I have approx 10 of my own site none of them affected in bad way so far, I submit an article for them every 7-15 days...
Also though I submit 2000 article in 4-5 hours they don't go live immediately. Around 10-15% of them gets auto approved. and even if they are live Google does not index them instantly.
So far I have done around 500 submission since I started providing this service including my own Sites, and none of my site or my client site has affected in negative manner or sandboxed...
It Is working Positively....