IMO there seems to be some consistent problems here. I am still not convinced that there is a penalty or not but for me too many people seem to have blindly signed up to networks and not done any due diligence?? So for some people (not all) they only have themselves to blame.
I have seen some networks that are poorly set up, so just a massive block of text with links imbedded or just lots of posts with a single link for each subscriber. Really you should be checking for things that I believe (with experience) that will stop sites from being deindexed.
Here are a few rules you should try to stick to when signing up to a network (IMO)-
1. Make sure you get a report (you need to be able to do your due diligence ASAP to protect your sites)
2. Make sure the sites are media rich (images & videos)
3. Make sure OBL's are low (not 80 or 100 like a lot of networks - below 60 is a lot safer)
4. Make sure anchor text variation is allowed
5. Don't go too big (networks with large numbers of sites will leave a bigger footprint, especially if they limit keywords/URLs too low)
6. Make sure the site has some authority links
7. Check to see that some posts have no links (having every post with 1 link is a little obvious)
8. Check the work count on posts (for example if a network puts your link in a 250ish word post then again a load of posts with similar word count is another big footprint)
I hope this helps all those who are still looking into networks as THEY STILL WORK
I have some networks that are still doing fine, the only problems people have had is when they are members on other networks that are poorly put together, most members still have great rankings so don't write them!!