Visas cost 5-10k per founder. We had to pay an extra month of rent as 'deposit'. You will need to fly in and out of the US several times while the visas get process, as the ESTA is only 3 months. A round trip from LON-SF can be 1k per person, and that's from a hub. I've already got enough miles to be bumped up on my frequent flyer prog.
It's not the end of the world, and the typical YC company will be resilient enough to survive whatever.
However it does add up fast (40k easily), it's not something you can avoid, like say buy a less shiny laptop, and reduces your runway in a meaningful way.