Sword of Spirit
Legend
I think the problem is that attacks scale in power differently for different classes. One good illustration is the fighter and the rogue. The fighter, largely, scales up its attack with more attacks. The rogue, on the other hand, keeps its one attack but it becomes more and more lethal over time. These new melee cantrips don't deal with those kinds of scaling in the same way.
Even at the cost of a feat in lieu of +2 to a stat, a rogue gains so much and sacrifices so little by taking one of those cantrips.
Sure, it's pretty good for a rogue. Probably pushing into the exploit territory. Also good for a cleric with Divine Strike.
Personally, if someone were going to take it with rogue for a mechanical super-damage concept, I would strongly discourage it, but they likely wouldn't be playing in my game anyway because my players prefer not to optimize significantly above the level of the rest of the party.
But anyone who wants to play a 3e-style lightly armored rapier bard can now take Lore bard and use Additional Magical Secrets to pick up greenflame blade or booming blade and actually be primarily melee. Before they came out they were truly stuck with a design that wouldn't allow a primary melee build. Unfortunately, the one other subclass that had the exact same problem (Land Druid) doesn't derive as much benefit from it, because they need a feat to get it, and I really think that is asking too much to allow a druid to choose a combat style other than poor ranged attack cantrips or turning into a bear.