Updated February 23, 2019: added links to the different pledge manager sites.
In looking up different pledge managers on my least favorite search engine, I came across this article: https://stonemaiergames.com/backers-decide-kickstarter-survey-vs-third-party-pledge-manager/. The article does a decent review, but leaves out one thing I have seen mentioned by numerous crowdfunding creators. Shipping fees, the percent creators have to pay to Kickstarter, tend to be higher than what the fees creators pay to pledge manager sites. He did a follow-up article, http://www.jamesmathe.com/story-of-the-3-little-pledge-managers/.
In the second link, he gives a great description of the three (3) pledge managers I am somewhat familiar with: Backerkit – https://www.backerkit.com/, CrowdOx – https://www.crowdox.com/, and Pledge Manager – https://www.pledgemanager.com/. He also gives comparisons between them in a color coded chart showing several funding levels.