It's an appalling contrivance - again, poor storytelling from JJ.Another reason I don’t like the film is how they made The First Order seemingly be able to conquer the whole galaxy off screen. Sure they explained it in expanded media, but it feels like lazy writing in trying to make these guys look stronger without trying.
So keen is JJ to recreate Star Wars (A New Hope), he has to magic up an excuse for a small force of plucky underdogs against a galaxy-dominating power - so the Galactic Republic is a problem that needs to be eliminated. Literally. The existence of the Resistance is in itself a contrivance, to sort of shuffle the Republic out of the way.
The way to do this, and again to oh-so-tediously ape SW:ANH, is a planet killer. However, the idea that The First Order carried out this truly colossal construction project (think of the manpower and resources!) without so much as a single spy noticing and reporting it beggars belief. Next, the eternal cliche that the next threat has to be even bigger and more destructive than the last. This eats stars for fuel! And it can blow up planets from light-years away with faster-than-light blasts crossing the vastness of space. Never mind that the Republic, very conveniently, held most of its fleet at its capital system so the whole lot could be destroyed in one go. What a slushy mess.