Are you claiming there is actual guarantee for this when using Mixtum, and there is no such thing for Unijon?
I never had problems with any coins that came from Unijon service and I use it for privacy, not to make my coins any cleaner.
I will repeat with a bit more details.
Mixtum as a partner receives coins from Jambler. Jambler has strict control of Bitcoins that are mixed and come from investors, if they do not meet the expected "cleanliness" level, they are returned to the investor. So there is a serious analysis before the coins are sent to their mixing system.
Unijoin (like most other non-Jambler mixers) does not have this kind of control over every coin that comes into the network.
That's the difference in theory. To prove this theory, at least 30 transactions should be done on each mixer and then compare the analyses.
As with other things, no one can guarantee anything, but a better understanding of the process will definitely help.