Happy IBKR customer here. ForecastTrader has absolutely horrific liquidity outside of maybe 30-40 large contracts. The rest is all market makers that only offer 10-100 or so shares at each price point before bumping up a penny or two. No knock on IBKR as a whole, but you can't even effectively buy on most events or outcomes without slippage eating away your entire edge, and forget about real serious positions above a few grand entirely outside of those 30-40 big contracts.