The 3/3 body is a tad small to risk regularly in combat, but you can always send your other Frogs (or changelings) to earn the trigger or just use one of the other spells from this list 11
The Goat version gives you one lore per trigger, so you'll want to bounce Merlin in and out of play as much as possible
You can find more of my musings on twitter @travishall456 And you should check out other takes on Bloomburrow's top cards here: Top Ten Cards from Bloomburrow #1 - Abe Sargent The Top Five Cards For Each Color in Bloomburrow - Ali Aintrazi
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He is very good, and very rounded offering a wider variety of abilities that work well in any deck and as a result I find I play him in almost every green deck I build and 100% of the heavy/mono green decks
You get the same chances of pulling the rarer cards from them all except with the two Starter Decks