Not an expert on the Iberian case, but surely the cap just ended up topping up gas-gen (paid by consumers) and create distortions -> increase exports (who enjoy the artificially low price) and increase gas use by moving it down the merit order

I’d add that marginal pricing isn’t really a design choice but just the natural way prices form in commodities markets, providing essential operational and investment signals. Moving away from it would be v complex with unclear benefits to consumers (hence why it was ruled out)