I don't recommend using an adapter for anything other than a temporary situation.
Now, the main question is: Is this safe?
The problem here is that manufacturers blindly apply equivalency tables when the shouldn't.
Picture a more common situation - a side table lamp with a small shade. Typically you'll see a warning sticker saying "Don't use a bulb greater than 60 watts/9 W LED."
This is due to a mistaken idea that an incandescent and an LED are in all ways equivalent. However, they are not. An incandescent bulb is about 2% efficient in that it's output is about 1.2 Watts of light energy, and 58.8 Watts of heat and IR energy. It is this heat energy which is the reason for the safety sticker. More heat output, and you start to melt or burn the shade, which is a fire hazard.
Now, without getting into the efficiency of LEDs, it's obvious that 9W is significantly lower than 58.8 Watts. That is, there is no possible way the LED bulb can produce enough heat to damage the lampshade and present a fire hazard.
For completeness, we should also discuss the input side. Obviously, putting in a megaWatt bulb of any sort will, in normal circumstances flip a breaker or blow a fuse. In abnormal circumstances, it can melt the wiring in your wall or the internal workings of the socket and any integrated switch assembly. One would hope that all lighting fixtures could handle any reasonable load, with cheaper and cheaper products on the market, there may be smaller gauge internal wiring or other components that might not be able to handle the current draw of a 150 w bulb, thus the warning. (But, the main reason is still heat and fire.) But, even if this is true, 9W is well below the spec, and it is safe to use a 9W bulb in a fixture designed for a 60W incandescent.
TL;DR: If a fixture is rated for a 60 W bulb, any bulb of equal or lower wattage (within reasonable geometry) will be safe in that fixture.