I was looking at the statistics of my current game today - I had beelined to Privateers and wanted to see how much effect they had had (aside from getting a few Generals).
None of the AIs have Chemistry yet - curious, because some have Physics and many/most of them have Astronomy now. But the numbers look like this:
My losses: 6 Privateers
AI losses: 3 Galleons, 34 Galleys, 81 Triremes, 197 Caravels.
Now, I wasn't attacking with even moderately wounded units, and only a handful of attacks (probably those against the Galleons) were with worse than 98% odds. Usually it was 99%+. So my 6 lost units versus the 315 lost by the AI sound about right. Taking into account the couple of occasions where I foolishly left lone wounded Privateers exposed to attack, the system seems to work.
I have won a few times with low odds - I suicided my last attacker against the final defender of the last Portuguese city earlier in the same game, and won with 20% odds - but I do that so rarely that I can't speak to the accuracy of the odds on that end of the scale.