The answer depends on the type of cut. If you have a tender cut of meat then there's no reason to cook it any longer than then desired doneness. If you are using a tough cut then there's lots of collagen that needs to be broken down, and that requires moisture and time. You want to cook it until all the collagen is broken down as that will make the meat tender. That may take 2 hours, it may take 6, it depends on the cut and the thickness.
I have a similar experience with my slow cooker, it goes way too fast. To get around this I've ended up putting in big chunks of meat and veggies because anything else is completely nuked after 8 hours! I've also thought about using a light timer to have it start 3 hours after I leave home.
It sounds like you were expecting slow cooking to be like sous vide. Well, it's not. The point isn't controlled sub-boiling temperatures, it's something on the border between simmering and boiling for foods that just need a long time to cook at that approximate temperature.
Slow cooker recipes are not supposed to be very sensitive. They're expecting to be approximately boiling for most of the time, and the difference between low and high is pretty much whether the boil is marginal or a bit more substantial. In many cases, this just matters because the quantity in the pot varies, and it takes more to keep the stuff at the top hotter if it's farther from the bottom, or if it hasn't all cooked down into the liquid yet. In either case, it shouldn't be a full rolling boil; it's just boiling on the bottom, so the rest of the liquid is probably a bit below the boiling point. And yes, this is still slow cooking. It's not boiling fast enough to lose a huge amount of liquid (or worse, boil over) with the lid on.
I wouldn't really try to assign temperatures to slow cooker recipes. Like I said, they shouldn't be that sensitive. If your bean soup recipe didn't work, maybe it was a bad one. But "reliable" in the context of slow cooker recipes doesn't mean "exact times and temperatures". They're generally things that will be perfectly fine if you cook them 25% longer. Not everything in the kitchen has to be precise and formulaic; slow cookers and slow cooker recipes take advantage of that fact. And even if you do try to calibrate, you'll have trouble, since there's a temperature gradient from bottom to top. Unlike sous vide, a slow cooker is not constantly well-mixed. In equilibrium it'll be boiling at the bottom, and 10-20 degrees cooler at the top.
If you really wanted to use a sous vide controller, I imagine something like 95-98C would work for basically every slow cooker recipe, no matter whether they say high or low. Of course, mixing thoroughly enough to make your controller actually work, you may be overstirring whatever you're cooking. But the point is, things you cook in a slow cooker aren't really going to care much what the exact temperature is; it just matters that it's hot, near boiling, and not boiling so fast that it sticks on the bottom or loses a lot of liquid.
Finding reliable recipes... Well, it's like anything else. If you're looking on the internet, you have to learn to judge for yourself and look for warning signs, or stick to sites with lots of reviews. You also have to accept that sometimes you have to test for doneness and be flexible about time. This isn't really unusual; baking recipes should always have some kind of test ("until golden brown") and the actual baking times will vary. (With something like bean soup, sure, maybe the recipe was bad, maybe you didn't soak enough, maybe the beans were a little different. A stovetop recipe wouldn't have been precise either.) If all that isn't good enough for you, buy a slow cooker cookbook; tons of those have been published in recent years.
Best Answer
It depends on the particulars of the recipe and the ingredients and the slow cooker itself.
You say you are using chicken or beef, but are the pieces of meat small or large? Is the meat lean or fatty? I've had problems with overcooking large pieces of lean meat, such as chicken breast, but smaller pieces and/or fattier pieces seem to hold up better to extra-long cooking.
Another issue is proportion of liquid. For example, soups tend to work better in your scenario than "drier" recipes.
And, of course, the slow cooker itself can make a difference. I assume you are talking about 9-10 hours on low, not on high. But low itself is not likely to be entirely consistent between cookers. For example, a large slow cooker is likely to put out more total energy than a small slow cooker, so for extra-long cooking times, you are probably better off using a small cooker with a full pot that a large cooker with a half-full pot.