You might discover that the less than/greater than functions only work with numbers (not dates), so you might want to retool to say something like "if the difference between these two dates is less than (or greater than) some number".
e.g. IF(WEEKDAYDIFF({DE:SLA Date},{actualCompletionDate})>0,"SLA MET","SLA Not Met")
So basically think up some equation that results in a number being greater or less than some other number.
And, I would also double check to make sure your initial calculated field is formatted as a date field.