My company uses Brex among others. They do nightly sweeps into a set of partner institutions on your behalf. In theory means you are covered for $250K * number of banks, but the scheme is pretty opaque. It's also untested by a real failure as far as I can tell. The best mitigation is to spread accounts across multiple institutions.

p.s., Brex works fine for day-to-day transactions like AR, AP, and Payroll. We've been quite happy with it, but these days you can't fully trust any financial institution.