I would say it's rubbish just because it really can't paint well for itself. In order to utilise Charge Keep, you need some actual turf in the first place, and it's hard for this thing because of how long it takes for such a short-range shot. While you can always just spam tap shot, it's still not all that effective.
The only real utilisation for its longer charge keep time, as mentioned above, is you can climb higher walls you couldn't with a splat charger. Ride some inkrails a while maybe. Its TTK is pretty godawful too, when you consider the full process. With its terrible charge time, you're not going to want to charge it out in a fight unlike the squiffer or Bamboozler can. So you back off from the fight, hide behind cover, take the 20 seconds to charge up, store your charge, swim back into the action, then use the shot. And if the shot misses, time to go back home to pick up another one -- if you survive. It's so convoluted and inefficient that I just don't think it's worth it.
Maybe it's just me, but I also greatly dislike its kit. Suction bombs feel terrible compared to Splat Bombs when it comes to flushing out blind spots chargers can't hit -- and if I'm using a Goo Tuber, I don't want to be close enough in the fray for Splashdown to be effective.