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I think that such a practice can be quite good if you do it well and if you have a good product or site to be reviewed. You don't even need such platforms to do that. We call it Blogger outreach campaign: Simply shortlist a few hundreds (or even less) of good blogs in the industry and market you want to focus on. Think of a what offer you can give to them. Money most probably won't work. You can incentivise them with an affiliate offer or something really touchy and personal. Put that in your email to them or on their contact us forms or call them. You will see that most of  the guys will reply and will be happy to cooperate. This works much better than any platform if you are going for the quality links not for the quantity.

I've seen nofollow in a bio at the bottom of guest articles, it can happen

JC Pennys got in trouble for this. Matt Cutts manually deranked them: http://searchengineland.com/new-york-times-exposes-j-c-penney-link-scheme-that-causes-plummeting-rankings-in-google-64529 Just don't overdo your act. Throw in variations of the keyword. Try and make it look more natural. Also, use your Google Analytics to find keywords that may actually be converting at a higher rate.

I had a look into this at one point, but then google was saying set your 'View All' page as your canonical, but our page 1 is already well indexed. I think you sohould only use these tags if google is having a problem with the strusture of the pages and if it is showing maybe a page for your keyword.