Lemons, Limes, Vinegars and other acidic products tenderize the meat by denaturing the proteins (breaking them).
Yogurt, Ginger and Papaya have enzymes that tenderize meat as well.
I am normally more concerned about marinating meat in a lot of ginger because it destroys the texture and you can at that point throw away the meat. You can tell I have made a few mistakes when I was learning how to cook
For something like chicken legs and thighs you can marinate them in lemon overnight and it will really not affect the meat. I do this all the time and in fact the flavor is more intense and the end product more tender and delicious. For tender cuts of meat like breasts you should try to marinate them at the most for 4 hours in lemon. For fish and shrimp even lower.
An example is ceviche where they marinate fish (cut finely) in lots of lemon juice for several hours (8 is what a restaurant waiter told me). When I ate it, the texture of the meat was similar to a cooked fish. I guess I could not tell it was raw.