What are the downsides to "low and slow" when cooking meat?
The big downside, of course, is time. You don't always want to spend all day cooking something.

For relatively thin pieces of meat (a steak, a chicken breast, a fish fillet) there's just no need to take a long time making it. Worse, you risk making it so tender that it just falls apart. Pulled pork is a wonderful thing, but some days you want slices.

Other cooking methods can add more flavor. In addition to the Maillard reaction, flavorful fats in sautee can bring flavors of their own. Cooking on a grill introduces smoky flavors. 

Low and slow maximizes tenderness, but there are other desirable textures. Frying brings crunchiness, which you can't get from low-and-slow. Baked fish takes on a firm texture. And so on.

Low and slow is a crucial tool to have in your arsenal, but it's not the only technique you should learn. Some days you just want to slap something in a pan.