No I am not wrong. There are even more problems besides the voltage/current interface issues. These DSR/RTS signals, etc... How the USB driver "simulating the RS232 port" deals with that. The problems like I said, are legion...
My recommendation is to buy a RS-232 card and plug it in!
That is the simple answer.
get more complex only if you have to.
These USB<->RS-232 things work well when the flow control is X-ON/X-OFF. They do not work well when the "extra" RS-2332 lines like "DATA-SET-READY" (DSR) are used for flow control (or other out of band signaling ). I believe that to be true and have seen it. If there is just "in-band" signaling, like X-ON/X-OFF, everybody is happy. Once we get into the realm where the hardware flow control is "out of band", trouble starts. Sometimes these signals aren't even used for hardware flow control and are used for other signaling purposes. The USB thing often just "augers into the ground". It does work for X-ON/X-OFF and I'm not saying that it doesn't.
I stand by my recommendation of buying a card that will come with software drivers and only cost $30!
If the device only needs in-band signaling X-ON,X-OFF, then buy the cheapest USB<->RS232 thing that you can!