What is wrong with last-click attribution?
It gives all credit to the final touchpoint, systematically overvaluing bottom-funnel channels (brand search, retargeting, direct) and undervaluing the marketing that created the demand.
Last-click attribution credits whichever channel the buyer touched last before purchasing. That is usually a navigational visit - brand search, a direct type-in, a retargeting ad - rather than the channel that introduced or persuaded them.
The predictable outcome: budgets drift toward channels that harvest existing demand and away from ones that create it. Better options include multi-touch models for journey insight and incrementality or geo experiments for causal truth. Any model is only as good as the tracking underneath it, which is why signal recovery comes first.