I've tried onboard audio on several machines old and new, and the analog performance varies. Sometimes there is electrical interference noise in the playback, the recording, or both. Sometimes the playback leaks into the recording. Sometimes there are glitches. But in most cases, it's fine.
I'm not a hardware engineer but my guess is that the usage of the onboard audio chip is critical for good analog performance. Maybe the noise comes from poor shielding, poor components that it's hooked up to, bad design that is generating more noise than other designs, etc. All I can say is that sometimes it's there, sometimes it's not.
My recommendation to people these days is to try the onboard audio first. If there's noise or other problems, then get a sound card. Most modern games do all their mixing in software, plus modern Windows does everything in software, so hardware audio features are useless to most people nowadays. It just has to be a dumb pipe, but a clean one.