Anything possible to do wrong, was done wrong with the Sportsters. It is certainly no surprise that when US Robotics released Winmodems, that they did so in the Sportster line, and NOT in the Courier line. Like water, a modem is the last thing you should be sticking inside your computer case. Internal versions of a modem must consume a slot in the computer. Often, the serial port UART is combined with the modem electronics on a single chip, and the UART can be rather buggy.
Internal cards can and do conflict with other cards in the computer.
However, the worst part of an internal modem is the lack of diagnostic lights. Unlike most computer peripherals, the Send and Receive lights on an external modem are there for a reason, and provide invaluable troubleshooting information. I'll be covering how to interpret the proper light patterns on external modems later in the series. The thinking goes that the CPU has so many extra CPU cycles that there are plenty left over to spare for the modem. This is supposed to make the modem cheaper to manufacture.