EXSYS provides multi serial-port adapters connected over USB. I recently ordered the 8-port device EX-1338HMV. From a hardware perspective, the device is made very solid, ready for industrial use. The connectors, USB as well as power, are secured by additional screws, so tensions on the cables should be an issue.
As soon as I connected the USB cable, the serial ports worked without using any external power adapter, however depending how much the connected device actually draw, external power might be needed.
The device has FTDI chips inside, which was the reason I opted for this solution. FTDI chips have a good reputation in providing high quality USB-to-serial adapters with high accuracy. The FTDI chips are connected through two internal USB hubs, as can be shown using lsusb:
/: Bus 03.Port 1: Dev 1,, Driver=xhci_hcd/4p, 480M
|__ Port 1: Dev 14, If 0,, Driver=hub/7p, 480M
|__ Port 1: Dev 15, If 0, Specific Class, Driver=ftdi_sio, 12M
|__ Port 2: Dev 16, If 0, Specific Class, Driver=ftdi_sio, 12M
|__ Port 3: Dev 17, If 0, Specific Class, Driver=ftdi_sio, 12M
|__ Port 4: Dev 18, If 0, Specific Class, Driver=ftdi_sio, 12M
|__ Port 5: Dev 19, If 0, Specific Class, Driver=ftdi_sio, 12M
|__ Port 6: Dev 20, If 0, Specific Class, Driver=ftdi_sio, 12M
|__ Port 7: Dev 21, If 0,, Driver=hub/4p, 480M
|__ Port 1: Dev 22, If 0, Specific Class, Driver=ftdi_sio, 12M
|__ Port 2: Dev 23, If 0, Specific Class, Driver=ftdi_sio, 12M