Ok, I'll try to explain this as neutrally as possible.
HID capsules have a focal length. They are designed to be used in properly designed projectors.
Halogen bulbs have a focal length. They are designed to be used in properly designed housings.
The OEM lens are fluted design. They are designed this way to throw light in a specific pattern that is reflected from a specifically designed reflector. The fluting is part of the focusing.
HID lights are designed to project through clear lens. This is because the focusing of the capsules output is done inside the projector housing and lens. All the clear cover does is keep weather out.
Both are different in both design and function.
When you put "plug and play" capsules into housings that are designed for halogen bulbs, the focal length is wrong, the reflector design is wrong and the lens design is wrong.
Throw in the fact that no reputable HID capsule manufacturer (OSRAM, Phillips. etc) makes 900X based kits/capsules or anything besides 4300K capsules and you can be assured that anything "Plug and play" is manufactured in a china sweat shop. IE: no QA what-so-ever. This means there's light flying all over the place from one capsule to the next. I've had the china capsules, they're crap. Even the knock off ones that actually have 8122 or Phillips stenciled on the sides. Built crooked, different lengths, etc. Basically, nothing is right about them other than the fact that they power on when you hit the ignitor...
The result is light in areas where it should not be. Specifically, above the point where low beams are meant to project. Compounding this issue is the light output from capsules are more intense than the original halogens.
What results is what is commonly called "glare". What it officially is, is illegal. Modifications to DOT approved light assemblies are not allowed by law. That includes bulbs not OE spec for the housing. The light above the "cutoff" and it's resultant effect on drivers is the issue.
There's a good reason for it being illegal: it's blinding to oncoming traffic. While someone may be able to see if they squint or shield their eyes, their vision is impacted to the point where it is severely reduced by said squinting/shielding/lens constriction, making far more likely they will not see something in their way or loose proper guidance of their vehicle (like it or not, accept it or not, people tend to drive at bright lights, just like moths).
Compounding the issue is that while they were exposed to this brighter than normal light their night vision was severely reduced. It can take up to 20 mins for full return of normal human night vision. To make this even worse, older individuals take even longer to recover night vision, the road is shared with young and old drivers alike. During this 20 min period, drivers can miss shadowy things on the side of the road like signs, trees or that animal in the ditch ready to dark across the road. Even if we take night vision out of the argument, once a driver is exposed to a brighter light their eye constricts to limit the incoming light. Once the light passes by, their eye takes time to dilate again so they can see what is behind the vehicle with the brighter than normal lights. During this period, they can miss something behind your vehicle. Once again, the older you are, the longer it takes to recover normal vision.
Truth of the matter is, you are better to have full night vision when driving and low illumination (physically, not low output lights) of things far out in front to or off to the sides of you. The human eye is very good at picking up things at night if you are not looking at them directly in the dark. even better at picking out motion in the dark. It just how nature designed the eye (rods and cones and all that jazz). Light splayed all over the place reduces this ability by impacting your night vision.
Ideally, what you want is light down low and you looking over it in the darkness. This allows the light to illuminate objects and maintain your night vision eyes to be more sensitive to things not caught directly in the beam.
OEM HID projectors have a cutoff shield in the low beams for two functions: to prevent blinding of on coming drivers and to allow you proper vision.
So, while oncoming drivers are not "flashing you" (which is also illegal BTW), that doesn't mean you are not impacting their safety or those around them.
Now, I have HID's in my truck. In both high and low beams. However, my low beams are retrofit with a set of Acura TSX projectors with proper D2S capsules burning 4300K. They are grafted into aftermarket housings with clear lenses. While mine are properly aligned, have a cutoff like a razor blade and bother no one, they are still illegal. But I have at least made the attempt to be as close to the law (and social responsibility) as possible.
My high beams are plain old plug and play capsules and are nearly useless other than to light up road signs a bit better above my TSX's cutoff. They don't project anywhere near as far as my low beams. They are only used when no one else is on the road, which happens 90% of the time on the highways where I currently live. They are also only temporary until I can muster up enough cash to retrofit the high beams with projector lenses appropriate to HID high beams.
Even though I have gone through the effort to retrofit proper projectors in to my housings, they are still illegal by letter of the law. "no Modifications to approved light assemblies". But, by the same token; I have several friends in the RCMP (occupational hazard). They have seen my truck up close and we've talked about the lights. They agree that technically they could ticket, but they wouldn't even look twice at my truck as it rolls by because the lights are not offensive. Even though they have black inside and are obviously not stock, they pass by looking like OEM lights. These same officers also have said they pull over lights that look "brighter" or "bluer" than they should and ticket for illegal hid's all the time.
Oh, in case you think I don't know what I am talking about: I'm a military helicopter operator. I fly SAR and have in excess of 500 hrs on NVG's alone and somewhere north of 1000 hrs night ops. I am a night vision goggle instructor and have been for many years. You could say I have just a little experience with lights, night vision and the effects of it all together....
The bottom line is plug and play kits are crap. Cut off shields and all manner or workarounds help to some degree, but only cut down on your light output or mask the fact that reflector housings are not giving you the performance out of those capsules you could get.
If you must have HID headlamps, at least make the attempt to do it right and retrofit some proper projectors in there.
I'll get off the soap box now......