Okay, I think we're both being disingenuous if we're trying to compare Serum Powder to Brainstorm. They do different things, serve different roles, and are used in different decks. Just as having Serum Powder in play or in your hand won't let you dig for an answer to your opponent's turn six Yawgmoth's Will, Brainstorm will never attack for three with Karn.
Serum Powder is a free, uncounterable, one-sided draw 7 that can be played on 0. It also functions as a Mox while in play and better than a Mox while on the stack.
Serum Powder's lone drawback is its casting cost of 3 which is heavily mitigated when you have a Mishra's Workshop in play.
The thing is, I don't want that in my deck.
I think Serum Powder is a great card for decks that want to be at their finest in the early game and expect to end the game early on a regular basis. Especially game one, if Ichorid gets a Bazaar in its opening hand, it is likely to win the game handily. Likewise 2CM can expect to end the game quickly with a good hand; Serum Powder helps put one of two two-card combos together in that deck.
MUD is simply not that kind of deck. You're much more likely to have to grind out a win, especially if you draw cards that don't do anything for several turns in a row (e.g., redundant Crucibles, Null Rods, lands). To play Serum Powder, you're necessarily cutting either more useful artifacts or more useful mana. No, Serum Powder isn't a complete blank, but nine times out of 10 I would rather have it be almost anything else (the 10th time is when I need the non-Shop mana to activate something, in which case drawing a land would have been better since it can't be countered). At least then I can feel like, "Well, this card would have been useful in some other situation, but it won't help me here," rather than "Ugggggggggggghhhh..."
If you keep a seven-card hand with Serum Powder, great. Serum Powder could have, again, been a more useful artifact or another land. Mishra's Factory, for example does many of the things that Serum Powder does: produces mana, is non-Workshop mana, attacks, blocks sacs to Smokestack, taps to Tangle Wire, etc. It's also better in multiples. If you're already playing Factories, why not consider Mutavault? Or Ghost Quarter, Bazaar (also helps find important pieces!), or Rishadan Port. If you want more artifacts, you could be playing Sculpting Steels or Pithing Needles or max out on Null Rods and Crucibles. If you aren't running Null Rods you can play more Karn, Expedition Map, Jester's Cap, Sensei's Divining Top (also helps find important pieces!). After mulliganing, any of those sound better than Serum Powder.
If I was confident that a game would be over with the first seven cards in my hand, I would be happy playing Serum Powder. That's just not the case with MUD, though.