Well speed and reliability are a big factor with public proxies. Public proxies tend to go up and down much quicker than private due to the fact that they're public and most likely being scraped and used by a lot of people. Public proxies will not yield as many links as privates mainly due to reliability.
However, it depends on the public proxies. I've scraped around 17,000,000 + URL's over the course of a few days just using GSA Proxy Scraper + Gscraper so public proxies can be used for things like that.
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