I'm going to be controversial here and say that IMHO Shadow Peashooter isn't that great. It certainly does not live up to the hype. Mine is now at level 10, but despite that I'm still underwhelmed by it.
Okay, it has multiple abilities:-
[1] - Fires a laser once powered by Moonflower, but the beam isn't as strong as Laser Bean's.
[2] - Like Grimrose it swallows zombies that pass over it, but I've found that it more often than not fails to do so with fast moving zombies. EG Chickens, Charging Bulls, and irate Newspaper zombies. All it does with them is hide until they've gone past. During Battlez it sometimes even failed to drag slower zombies down.
[3] - It hides far too soon. As soon as a zombie even gets within the square in front of it, it stops firing and hides and becomes next to useless - especially when it fails to drag that zombie down. (It's MEANT to drag down up to 3 zombies when powered up, but that's doubtful - at least in my experience.) That makes it a plant that should always remain right at the back of any defence, where the zombies will not get close.
[4] - The Plant Food effect may look spectacular, but, like Strawburst's PF effect, it's more for show than for its effectiveness.
For the above reasons, Shadow Peashooter makes me think of the old saying that goes "Jack of all trades, but master of none."
As well as all this, I find that Shadow Peashooter makes the game-play slow down. I get it in both Battlez and Adventure Mode.
Because it fires so many projectiles Dusk Lobber is known for causing game slowdowns, but, at least for me, Shadow Peashooter is worse.
During its dedicated week I tried SP as a means of stopping the early chickens, mainly because it refreshes more quickly than Dusk Lobber. In as much as it DID stop many of them, I suppose it did its job pretty well. However, even having a few of them active meant I had to go to turbo mode to make the game run at a reasonable speed.
As the matches progressed I got more Dusk Lobbers out. Because Dusk Lobbers cover three lanes, rather than just one, I removed the few Shadow Peashooters I had planted. Lo and behold, the game got faster.
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No doubt others may disagree with me, but after all its hype I was expecting far more from SP than I got.