It's a problematic spell. I think greenflame blade* was intended as the "here's what we should have done with shillelagh answer"--though of course they apply to different classes.
Here's how the game was designed:
1) Every (sub)class has a built in method of scaling at-will damage as they level. Examples include things like Extra Attack, Sneak Attack, Divine Strike, etc. Complex examples are classes that split it into two features, like a paladin's single Extra Attack combined with Improved Divine Smite.
2) For some subclasses this is simply easy access to a scaling cantrip, or if they want them to do more damage there is a bonus to cantrip damage like Potent Spellcasting.
3) Some subclasses provide an option that is really only melee or ranged, and other subclasses provide both options.
Mostly, this works very well, but there are 2 subclasses that I feel received overly limited options.
Because they get no weapon attack scaling and the damage of vicious mockery is horrible, I assume that Lore Bards are intended to use Additional Magical Secrets to pick up a better primary attack cantrip. The problem is, before SCAG came out those were all ranged. If you wanted your Lore Bard to be a swashbuckling 3e style bard type, you were completely unsupported by the standard rules. That is, until SCAG came out and added greenflame blade as a great option (which actually risks overshadowing Valor Bard).
Land Druid is the second one with overly limited options. They basically are ranged cantrip users (and get no damage bonus for it, which really hurts their at-will damage compared to other classes). If you want to be a Land Druid with a quarterstaff--because, you know, that's kind of a pretty popular image of a robed spellcasting druid person--you get shillelagh as a consolation prize.
Unfortunately, that's all it is. While it's great at low level, once everyone else's at-will damage starts scaling, it becomes apparent that yours doesn't.
Sure, you can finagle more damage with feats and stuff, but you'll still be bottom of the barrel relying only on shillelagh if you are the Land Druid who it was actually made for. The best use of it is actually for, say, a Nature Domain cleric, since it stacks with their attack scaling and uses their attack stat, or an Arcana Cleric who picks it up with a feat and uses it with greenflame blade, or even better, a Bard who picks up both shillelagh and greenflame blade with Additional Magical Secrets and uses them together.
So the cantrip does not fill the role that it appears on the surface it is designed to fill--hence bad design, regardless of whether or not you can find a use for it somehow.
*booming blade is an alternative, but it requires more finesse