According to https://pae-web.presonusmusic.com/downloads/products/pdf/Interface_Comparison_Chart_06-17-221.pdf, the Studio 1824c uses USB 2.0. While I agree that it's too difficult to find that information, according to my calculations the USB 2.0 bandwidth of 480 Mbps should be more than adequate for the number of inputs and outputs, frequencies, and bit rate that the 1824c supports.
I calculate that 73.728 Mbps is about the maximum bandwidth the 1824c requires. That would be needed to support 8 inputs and 8 outputs simultaneously at 192 kHz. Each channel requires 192 kHz * 24 bits, which is 4.608 Mbps, and there are 16 channels.
Enabling more than 16 simultaneous channels requires lowering the bit rate to 96 kHz. You can then get 28 channels altogether, by adding 2 input and 2 output via S/PDIF, and 4 input and 4 output via ADAT. But because of the lower sample rate, this should actually require less bandwidth: 64.512 Mbps.
If you go down to 48 khz, you get 4 additional inputs and 4 additional outputs via ADAT, giving you 36 channels, but again the lower sample rate means the bandwidth requirement is even lower: 41.472 Mbps.
Even if you could use 36 24-bit channels simultaneously at 192 kHz, that would only require 165.888 Mbps, so I suspect the sample rate limits of additional channels are imposed by the S/PDIF and ADAT connections, rather than the USB 2.0 connection.
Given all that, I see no reason to lament the fact that the Studio 1824c doesn't sport a USB 3.1 connection.
