Prop Firm Spreads on $EURUSD / $GBPUSD — Hidden Costs?

Been looking at a few new prop firm options, moving past some of the older names. One thing that consistently catches my eye is the 'typical spread' they advertise, especially on majors like $EURUSD and $GBPUSD. Some are quoting numbers that feel incredibly tight, tighter than what I see even with some premium retail accounts. My question is, where's the catch? Are these numbers realistic under actual trading conditions, or is there a widening that occurs during volatility or news events that effectively negates the initial appeal? Trying to map out true trading costs, not just what's on the landing page. Any experienced views on actual execution costs and hidden slippage from different prop firm providers?