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So many complain over those encoders - but we used them from 2009 through 2015 with generally good results. yes sometimes not, these were usually due to either 1) contamination/ scratches of the metal optical target, or 2) associated with the connector, usually after an attempt to unplug and replug the connector - a delicate operation to be avoided.
they are a little fussy, prompting many to shift to other options (including us last year, not necessarily just the magnetic encoders, some go for Grayhill 63R's and another is the AM CIMcoder), but once they are set right up, they work fine. We still have some in inventory, so will use them at some point.