Voat can't yet handle large groups of people visiting the site all at the same time. This results in the site often being down. Many argue that the main issue is choice of underlying architecture (ASP.NET + MSSQL) that is difficult to scale.
While Slicer is a passion project for the site's only developer and there are no real issues currently, the rate at which the site improves and how it is handled is solely in the hands of one man. One point of failure may be an issue for some.
Frizbee is built around anonymity. The site automatically creates an identity for each visitor and the visitor can create multiple other identities freely. The site even offers an easy way to manage identities. These identities can be temporary or saved with a passphrase for later use.
Stacksity automatically embeds images and videos in the feed instead of requiring to navigate to a different site or having to manually click to show each separate item. It allows getting through a lot of images and videos quickly.