Let us assume for a moment that enthusiasm is a positive trait. After all, nothing difficult was ever accomplished without someone's intrinsic motivation to "get it done".

"Second to none" is a compliment. It is saying the same thing as "nobody is more enthusiastic than your are".

If, however, a person is pursuing a task with great enthusiasm but they are doing the wrong thing, such a comment might be made before or after whatever corrective instructions are offered. (This is the "sandwich" technique of negotiation: compliment, ask for changes, and compliment again.)

In that case, it is still a compliment, but consider it "punctuation" whether it comes before or after the corrective message - while it is a compliment, the corrective instruction is the "framed" message that is intended to be conveyed as well.