Do we have a good answer for using lower wattage bulbs in fixtures?

Question

"Can I use X watt CFL/LED/Halogen bulbs in a Y rated fixture?" seems to be a fairly popular question lately. Do we currently have a good answer for this question, that we can close new questions as a duplicate of?

Examples

Can I use a 60 watt Halogen bulb instead of a 100 watt incandescent bulb?
Can I put 42W CFL bulbs into a fixture marked as “R20 Halogen bulbs up to 50W max”?
Can I use an 11 watt bulb in a desk lamp that says “CFL max 9 watts”?
Can I use a 23 W CFL in a fixture that's rated for a 60 W incandescent?

Answer 1

All of these seem to be related to safety, not functionality and practicality. I would love to see more questions about the effective Lumens ratio between LED, CFL, Halogen, and incandescent, where certain areas of use are more or less effective, i.e., high ceilings, outdoor landscape usage.