Gulping Scraptrap

Most proliferate arrives on a spell or a single enters-the-battlefield tag: you fire it once, the counters tick up, and the card leaves the conversation. Stapling a second proliferate onto death is what makes this 4/4 worth more than its stats, because it pulls the trigger coming and going. Send it to the graveyard and it advances everything a second time on the way out, which means a sacrifice outlet can cash in the death trigger for a guaranteed second proliferate rather than leaving you a one-shot beater. In any shell built around counters (poison, +1/+1, charge counters on artifacts, loyalty on planeswalkers, or the -1/-1 counters that so much of this era's black cared about), it stops being a creature and starts being a disposable two-count proliferate body. The honest cost is that the payoff only exists when there are counters to grow: with a bare board, both triggers do nothing, and a five-mana 4/4 that traded its stats for a whiffed effect is a bad rate. That conditional ceiling is the design's whole balancing act. It scales exactly as far as the counter density of the deck around it, and idles at zero when that density isn't there.