This is a weapon that can do some solid single-round damage, but the damage output falls off sharply once you blow the first stack of charges. It isn't a finesse weapon, it doesn't have the thrown property, it only does 1d4 damage when you don't have charges built up — it's great in the hands of a non-martial class that needs to deal with the occasional enemy who punched through the front lines, but it won't replace a magic rapier for a Rogue or a magic longsword for a Fighter.

That said, there's a fair chance that some players will see this weapon and think they can cheese it by building up charges between encounters. But if they really want to spend up to 18 rounds bashing a wall after each encounter, just to pick up an additional 6d6 damage, they need to be prepared to deal with a level or two Exhaustion at the end of the adventuring day. And they still won't be doing as much damage as they could with something like a Flametongue Longsword.