I'll clarify, I LOVE my LED headlights. Considerable improvement in visibility, I've never had anyone flash their high beams at me, I've been on the receiving end of them (me in my Celica and my wife in the Outback) many times and they're completely acceptable in traffic. I don't love the 6000k color, but 4500k LEDs are CONSIDERABLY more expensive.
But, I tested them back to back, I make sure my lenses are clear (a quick buff every couple years keeps them looking like new), and aimed correctly.
I cannot imagine a situation where a well made and correctly installed LED in a halogen housing would scatter more light where it's not wanted than the halogen, as the light output is smaller. But, it is still important to note that they are different, and how they react with each housing will vary.
A friend gave me a set of cheap H6054 LED housings, because the light output was so awful. I intend to use them on my XT6, which is never destined for street use again (and may not even get headlights).
I was riding in another friend's Ford van recently, and he was bragging about how bright his new headlights were, but it didn't take long to realize they were casting light all over where it shouldn't be. These were cheap aftermarket OEM replacement housings with halogen bulbs in them. Worked out well for me, he gave me his old ones, they're just a little cloudy, and they'll fit my van, my dad's RV, and my in-laws RV...
The point is, the answer is not black and white. Some LEDs work great in some halogen housings, some probably don't. Some cheap reflectors/projectors/lenses suck, regardless of light source they are using or were designed for.