Brushes in UE4 are identical almost to UE3 in terms of why you don’t want to use them too much. All that is said about brushes here also went for UE3.
Performance aside there is also the very visible limitation of no smoothing edges.
Converting brushes to meshes should not be done! Unless you deal with very basic brushes (a cube). The outcome of that conversion would be terrible.
And indeed if performance is no problem, and you only have hard edges, sure brushes work fine and you can just carry on with them.