Yes, an auto-inflate jacket will automatically turn you face up, even if unconscious. Unfortunately, if you regain consciousness, you might be drowning against the side of the boat or watching it sail towards the sunset, depending on whether you were tethered. Unless you are in a high-traffic area, no one will pick you. The odds of your survival are remote at best.
On the other hand, if you are conscious, there are three probable scenarios. First, you might be pinned against the topside but still tethered. Assuming your tether was conservatively short, you are within reach of the toe rail. This is a rotten situation, but there are stories of sailors muscling their way back on board. It is frightening, but plausible for a fit sailor.
With a bulky inflated PFD, you can barely see where you are, let alone clamber over the rail. Even with assistance from the crew, a swimmer with an inflated PFD is greatly hindered. In our previous report on tethers, we recommended carrying a knife to puncture the bladder before re-boarding
Alternatively, you may find yourself swimming like mad for the last-chance-line, a towed dinghy, or even a channel marker. But swimming in an inflated PFD is next to impossible. The inflated bladder is also a hindrance inside a capsized hull.
With manual inflation you inflate only when you need flotation. During testing for the adjacent article, testers found that the type III PFD they wore for the boat brake test was a hindrance. It was worn only to simulate the added drag of foul weather gear.
Auto inflation is handy, but single handers need more control. Many auto-inflate PFDs can convert to a manual PFD with an inexpensive kit. Check into it.