set review of Kaladesh for Standard Pauper, I rated Self-Assembler as being among the best cards in Kaladesh and predicted that it was likely to make a major impact on the format. And as I might have predicted, this week I already heard one player requested that this card be banned, with the rationale that the card was being included in every deck and that each game seemed to come down to whomever drew this card first. While calling for such drastic action with only a couple weeks behind us is a bit premature, it's hard to fault these claims. Let's take a quick look at the card.
Squadron Hawk, an innocent looking Common from Magic 2011 that also fetched another copy of itself when it entered the battlefield. But this card is clearly much better than that. First, for only three more mana, you're getting a 4/4 rather than a 1/1 Flyer, which is clearly more relevant most of the time. Second, since this is a Colorless artifact, it can be played in any deck, and has additional synergies in the format. Third, while a vanilla 4/4 for 5 wouldn't normally make the cut in the format, it's still close enough to a full card in value to make drawing three additional copies of it over multiple turns very strong. In fact, it's a virtual three-for-one with the only drawback being you have to cast each one before you get access to the next one.
Unlike in Standard, our format doesn't include any means of dealing with multiple creatures at once beyond tokens, and also doesn't give you any means of removing your opponent's ability to play them. Really, the only answer in the format that doesn't involve massive card disadvantage is to counter the first one when it comes up, hopefully granting you enough time to finish off your opponent before he or she can draw the next one. Self-Assembler is almost always going to at least trade with any creature on the board, which means you don't really have any chance of holding it back with bigger creatures. So if you can't counter it, you're only other option is to race it, knocking your opponent down to zero before they can leverage the advantage gained from this card.
All that said, if you're not including this card in any deck that isn't super aggressive, you're probably making a mistake.