I have used PCMCIA serial card adapters and they worked reliably for me. This is a physical port unlike USB which is a software generated virtual port. As such USB is subject to possible interference as the posts in this thread attest. Serial-to-USB adapters can be problematic for the same reason.
My opinion is that the best practice is to have dedicated timing computers with physical serial port/s so that USB connection from timer can be eliminated. A good machine is a Dell D600 laptop. They date back to 2002-04 and have serial and parallel ports, USB ports, wifi and CD burner. They were made in the millions and leased in bulk to large corporations, so you can pick up good used models for under $100 on eBay. They come from the XP era, but with 1GB RAM installed, they will run Win7. Machines of that age will have dubious battery life, but Li-ion replacements can be had for under $30. It's not a big stretch for a ski club to purchase four of these so that both timing and race office have equipment that's not clogged up with family email and pictures and is guaranteed to have the right drivers and software.