An existing e26 base fixture will have a wattage rating for incandescent lights.

A 25w cfl will produce significantly less heat than a 100 to 150 w incandescent bulb.

However, if the fixture is fully enclosed or the socket points down, the bulb would probably not last very long as the internal electronics may fail.

Led lights emit less heat than cfls, but like cfls use electronic components that are likely to fail when the rated operating temp is exceeded.

It's about the heat. Higher output LEDs or CFLs (if they fit and have dimmer capability if relevant for the circuit) are suitable in fixtures designed for higher wattage (but lower output) incandescent bulbs.

The caution about premature failure of downward facing or enclosed CFLs is reasonable but is, in my experience, not always the case. I have a dozen downward facing enclosed Philips bulbs in recessed fixtures that have lasted over 10 years, and some are used all evening every day.