A danger of not having PCI Slots is if the built-in Ethernet or Audio Cards burn out you cannot replace it, and have to junk the Computer. Look into PCI Express Cards to see if any would work with FreeBSD. I’ll bet they’re Windows-only.
I think the situation is quite opposite. Those components integrated on the motherboard are usually connected using PCIe(xpress) themselfs. So if they work under BSD, you can go and buy their replacement in form of the standalone card. And driver quality and general availability depends on many factors. You can have cheap low end devices like Realtek ethernet or generic AC97 sound chips. Here you get what you paid for - HW bugs, SW bugs, almost zero manufacturer support, but those devices are everywhere, so there would be probably at least community authored reverse engineered driver. Then you have something like middle-class and specialty devices like industrial controllers. Quality is better, but penetration lower, so if manufacturer doesn't support your OS of choice, you are usually out of luck here. And finally, you can get some hi-end devices. Being it sound device, it may be a problem, because of low penetration of BSD, maybe even Linux between sound professionals and thus nill support from the manufacturer. However if we are talking about network adapters, it is absolutely different situation - be it Chelsio, Mellanox, Intel, all of them directly support FreeBSD. Some better, some worse, but there are dedicated FreeBSD developers in all those companies, some of them even have commit bit directly to the FreeBSD source code. And why? Because they have market here. For example Netflix serve about 40 % of the internet traffic and not from Windows boxes.
Thanks for these further details about PCI Slot access. This explains why US Robotics says that their Internal Hardware Modem still requires a Driver to work with FreeBSD – which begs the question whether it makes sense to sell an Internal Hardware Modem rather than just Software Modems. The Internal Hard Modem sounds like a Soft Modem.
Do you think it would be possible for BSD to have a universal Modem Driver which would work on all Hayes Compatible Internal (Slot) Modems?
Generally, yes. Would anyone invest in it? Probably no. What is problem here - using serial line for modem, you have defined both HW (RX, TX, speeds, meanings of signals, timing) and SW (AT commands) interface. If you remove this, you have to redefine both of them. You can either make something what would mimics the old external behavior (and at least some of the not-soft modems did that) or use/define new standard or define your own.
First solution - create card, which appears to the OS like some generic serial device and use AT commands. Some cards did that and there is a chance that generic OS driver for serial port would suffice.
Second solution - use or define and agree with others on some standard. This may looks like something unusual in the computer industry, but there are many examples - USB keyboard, mouse, flash drive, none of them need special driver, because of the standard. Before 2D/3D acceleration in the graphics you didn't need driver, because all cards supported VGA/VESA etc.
And third - you got signaling from and to the device using standards, PCIe here and you have to provide software to actually use your device, the driver.