Here's a slight spoiler for you. Spirit wielders actually operate at 140% of the power of their respective pokemon. (Yes, they are 40% stronger than just the pokemon alone.)
And pokemon are already operating on superhero level, only there are no deus ex machinas against pokemon like say kryptonite or whatever. So yeah... they're somewhat tanky by default. Consider someone like Lucy, who actually specializes in tanking. She could probably survive an infinite amount of explosives being thrown at her, even beyond realistic amounts of what someone could have. Hell, knowing her she'd eat one and merely find it spicy when it exploded. The explosives would just stop hurting at all after she used her defense boosting move.
Nevermind pokespirit wielders that can heal themselves, or the fact that most wielders are significantly faster than possible for any human while fused. (With... a few special exceptions) Nevermind that any human attack is going to be normal-type. Nevermind abilities and items.
Realistically speaking, you're not going to kill any spirit wielder with explosives. Distract? Not all that hard. Stun? Pretty plausible. Hurt? I guess. Wound? Maybe if they took a powerful, direct explosion. Knockout? Not likely, but possible. Kill..? Getting a bit ridiculous.
To put pipe bombs in perspective, people dying while making them is a pretty common occurance in real life, let alone the evolutionary pampered people of the world of Pokemon. There's an entire science behind trying to get them to explode correctly too which has a lot to do with pressure and how quickly the pressure rises.
When people were going to war in Fallout, there was really no reason for this. If they were going to invent something they'd probably be aiming for something that's actually a great deal more effective than a blade... like a gun. But yeah. Not happening. So.
Besides, they've got something akin to landmines. That's the most dangerous human weapon in existance in Fallout in the sense that they're not hard to use--They are hard to make though, so you won't see a lot of them. The underground is probably the easiest place to find one now that the wars have more or less stopped.
Well, it might be a way to actually kill a spirit wielder. Emphasis on "might."
I don't really have much of an idea how pipe bombs are made, but still, it's not as crazy as some of the things that I find in video games.
Take this in mind: In Fallout 3 & New Vegas, there's a weapon called "Fat Man." It's a pneumatic launcher that literally shoots out miniature nukes at whatever you're pointing it at, & since the blast radius of those are just ridiculous, plus the fact that the projectile has a parabolic arc to it, seeing that the weapon is pneumatic, it's way too easy to kill yourself using it unless you're far away from ground zero.
Probably... because their only competition is a race created by a divine being to serve them?
You can't evolve to become strong without legit adversity. ||P It's frankly a wonder humanity is as advanced as it is in Pokemon.
But really, do you have any idea how dangerous and complicated pipe bombs are? I mean, /honestly/. Why would they ever feel the need to go through the trouble of making something like that in the first place, let alone building it when they could just... y'know. Do something easier and less dangerous.
Hmm... you do raise a really good point. It could end up being more of a liability than an asset to them, especially against a fire-type spirit wielder. One well-placed Ember, & boom, you've got a bunch of limbless acrobats flying every which way.
Okay, how about pipe bombs? I'm not really sure if they've still got remote detonation technology, however...
Sort of, I guess. It would be very hard to amass enough gunpowder to justify much of a keg, but you could do it. It probably wouldn't even be as effective as a stick of dynamite or a landmine and would be pretty difficult to use even on humans, let alone spirit wielders.