Some of the most difficult fixtures to maintain and retrofit are in ground well lights. Typically you find these outdoors, but when you have a decorative fish statue like the above picture and it is surrounded in rock you might find a well light. Traditionally well lights have HID lamps (high pressure sodium or metal halide) and given they are in ground, they need to be completely sealed.
Going back to the first statement when you see these in the field they typically leak and fill up with water which means they need to be replaced/repaired often so many customers with these want LED retrofits in hopes of less maintenance. The issue with attempting to retrofit these fixtures is that water will damage the LED the same as an HID and more importantly being in a sealed beam fixture (or attempting to be sealed) means there is little to no room for heat to escape. Heat kills LED so a retrofit for this fixture is often not a good fit.
Depending on the application however you might be able to surface mount an outdoor rated LED flood fixture like the one in the picture above and do a better job lighting what you are trying to light while significantly reducing your maintenance costs.
To sum it up don’t go jamming LEDs in every fixture you can. Review the application and consider the best solution to solve the customer’s problem.