I tested many USB UARTs, some more expensive and claiming large FIFOs, only to find out that they all start corrupting data when around 1Mbps is exceeded. Not by much, but they might skip a byte every five to fifteen seconds, if full bandwidth is consumed.
I don't have an idea whether the problem was with the chips themselves, or broken linux drivers, but anyway that's what happened - they were all unusable beyond about 460800bps or 921600bps.
So I have trust issues when it comes to these adapters ran at high bitrates.