Let me introduce this review with one statement: This is a good game. It leaves a satisfying feeling upon completion. The multiple usable characters are also pretty nifty especially with the characters having varying skill sets! It was applied really well in the last level before P-Bot where you had to incorporate different characters' abilities to finish the level! Another thing I liked was the level design for all of the levels. While they weren't really extremely puzzling or dauntingly challenging, they were cleverly designed and they worked quite well in carrying the game concept.
You are partially correct about Salad Fingers not being able to damage P-Bot, but being partially correct means that you are also partially incorrect, I'll elaborate on this a little bit later.
But first, think about it logically. If you couldn't damage P-Bot using Salad Fingers then they wouldn't have included him in the party. It would be entirely non-sensical to have a non-functional character in a boss battle. If anyone here wants to read further, I will have to mention that there are major SPOILERS!!!!! ahead. I will first go on the specifics of Salad Finger's abilities then I will explain why ponxs is partially correct about his statement.
During the pre-P-Bot levels, I noticed that Salad Fingers "attack" is actually a defensive move. I first noticed this in the level where you get Salad Fingers. I too was wondering why Salad Fingers had no "attack". In fact, he actually had a useful defensive characteristic. If you use his "attack" it would cause normal enemies to stop attacking! This should serve as a hint as to what sort of strategy you should use when battling P-Bot. You probably already have an idea of where I'm going with this.
You can beat P-Bot using Salad Fingers by using his "attack" to deflect P-Bot's missiles and make those missiles hit P-Bot; a defensive tactic! Since Salad Fingers has no weapons, he uses a defensive shielding tactic instead, using P-Bot's own projectiles against it. Yes, technically, ponxs is correct, since you can't damage P-Bot with Salad Fingers, because Salad Fingers doesn't have weapons and what you're actually doing is damaging P-Bot with its own weapons, but technically ponxs is wrong too because P-Bot is conquerable with Salad Fingers, though Salad Fingers may not be doing any attacking, he is catalysing P-Bot's self-destruction.
Now back to the review. The game was well thought out for the most part, the ending felt a bit anti-climatic though. Yes, it was unexpected, and while unexpected is good sometimes, I wasn't entirely satisfied by the ending of the game. Despite that, the entire game as a whole package was really enjoyable, and I would recommend it to everyone in the NG community.