arnt moonbeams just reflected sunbeams?

That's a good question. I've wondered about the explanation of that, myself. I don't know the official answer - if there is one - but I have some ideas. On the tree of life (qabalah), the progression of the mystic is up the central pillar, from the physical world (malkuth) to the moon (yesod) to the sun (tiphareth) to the ultimate (kether, and beyond), which I like to associate with the stars (but that may be just me). Interestingly, the Italian Witchcraft tradition - which is one of the principal sources for Gardner's Wicca - teaches that when we die we go to the Moon, and from there are either reborn or (if we are ready) are sent on to be "forged by the Sun" and then taken to dwell among the stars "in our new divine form." The moon is "light in the darkness," symbolizing our guide out of ignorance. It expresses a profound truth about the nature of spiritual awareness: that we cannot rely on the mere power of our own reason to show us Truth. I believe I have read that the Moon changes the light of the Sun somehow, magickally speaking. I don't know the mechanics, but surely you can Google this question and learn something more about it. My practice is more devotionally than magickally oriented - though I'm not averse to spellwork, I just don't do much of it - so this question has been more one of casual than burning interest to me. Perhaps someone with more magickal knowledge will respond.

I think the polarity is changed in the reflection. Moonbeams most likely were involved in the start of life on earth also.

Sci. I'd say they are weaker due to being a reflection of the light that hit's the moon.

The energies of lunar light have a much more subtle, but deeper, effect on the psyche, the deep mind and the etheric body than do those of solar light.

You can't notice sunbeams as much because they happen during the day when everything is bright. But moonbeams happen at night and have to push their way through all the dark.

If you're talking in terms of witchcraft, I do not believe they are. Overall it depends on the type of spell/ritual you are performing, your intent, how strong of a witch you are, which aspect of the God/Goddess you are using (if any), and several other possible factors.