1. No, they don't have to make every weapon super effective on every map. That's the whole point of picking the right weapon for a given two maps, and the whole point of having weapon choices in the first place. Also, every weapon is effective in the right hands.
2. You're nitpicking on it's weaknesses while ignoring the weaknesses of the other weapons. "The sprinkler only is good when left alone", yea, that's why you plant them in the right spots. You don't just toss it in the middle of a crowd like a grenade. Just like the Ink Wall is somewhat ineffective against people who flank. Or the delay timer on splatterbombs before the explode. Or the power of the balloon bombs (I forget what they are called off the top of my head). The only real offensive ability of the brush is the inkstrike, the rest, it's a territory painter, and a fragile speedster.
3. I've gotten 1-swing set kills with it without a problem. Not sure how you aren't. Perhaps you need to center the enemy with the swipe and not just splash spam like rollers. However, I've found it more effective to pop up, swipe, drop, pop swipe, to keep them guessing and inaccurate.
4. You're complaining about a weapon that nets you "1100-1400 points" when you use it your way... That's not underpowered or ineffective. You just might feel it takes more effort as it contrasts with your usual playstyle when using a different weapon.
5. You don't 1v1 with this weapon. Charging in solo 1v1 with the brush is not how you use it. If you do, that is an easy kill and you're being stupid. A main part of this game is know when to run. You dash in to an area where there isn't a firefight going on, paint it up, then zip over to the next before the enemy can hit you. Then, you move to the area where they left, cover it up with ink, then zip out of there. You control enemy movements with this weapon by directing the crowd without being seen or confronted, while covering up an area for your team.
6. No, not every weapon is a cover with ink quickly weapon. The Charger isn't, even on corridor maps. It's a kill weapon. The gal weapons aren't very good at spreading. They are kill weapons. Anything that is a sure 1-2 hit kill are either only close range, or not good at spreading ink. (Aero, not even 1 hit kills, but fast enough fire rate = close range, roller = close range, gal = not good with ink, snipers = not good with ink)
7. You pointed out yourself that you don't want to have to pick a weapon and only use the one you want to use. That's like saying "I don't want to use an AR on this wide open map. I only want to use my shotgun! The AR is OP, and my SG needs a buff since it can't compete at range". All weapons weren't made to be good at everything. If the game was completely 100% even, there would be no weapon variation. Everyone can only use the Splattershot. However, in this game, you always have a choice, and because you have a choice of weapons, you are always going to be stuck with a dilemma. You can pick the best weapon for a given stage, or pick the one you like to use the best. That's how every shooter game has been. If you want a weapon that can do everything at once, pick an all around weapon, like the Splattershot. It outranges some weapons, can kill faster than some, can spread ink quickly, has a good variety of specials, and can be used close up or at a distance. To use other weapons has some give and some take. You pick which features you want to focus more on (speed, ink spread, ambush tactics), and decide to neglect the others (range, power).