Users of SpamSieve who loved it report that on upgrading to Sierra it’s now terrible at stopping spam. I assume they have to retrain it. Myself I’m testing it, on their recommendation to see if the whole organisation should adopt it. Sadly it’s only correctly stopped one single spam email so far. The training seems very slow, or maybe it simply doesn’t really work?
SpamSieve is fully compatible with Sierra, and there are no known reasons that the accuracy should be any different from with other OS versions. However, it is somewhat common for Mail to lose some of its rule settings when the OS is upgraded. If SpamSieve’s rule is deleted or unchecked, that will have the effect of turning off the filtering. Please see this page for how you can check the Mail setup.
Retraining is usually only necessary if SpamSieve’s data files had been deleted or if you had stopped correcting the mistakes.