Maybe they are aimed a little high. I have HID in my low beam and LED in my high beam. In the rain it seems like they are dim but I notice the street signs way on down the road glowing from my headlights so I know they are working.
The low beam HID's shine out a good piece in front of the car. I ended up aiming my high beam LED's down some to supplement the HID's as I drive out the ridge scanning for the many many deer in our neck of the woods. My LED highs were shining about as far as the HID lows. My HIDs are not aimed too high and people don't flash me to turn them off.
Also it could be the temperature in kelvins of the HIDs. 6000K is the brightest white light. As you go lower like down to 3000K they start getting a yellow cast like fog lamps. Going up higher from around 6500K the light is a lot more blue up to around 10,000K. Blue might look good but in the rain they are almost non existent as far as usable light.
I doubt that it is a weak ballast because they usually die rather than get dimmer before death. Also the HID bulbs last a lot longer than halogens. HIDs are not good for high beams because they don't take well to being flashed as it take more voltage from the ballast to "start" the bulb. Whenever I turn mine on I leave them on for a couple of minutes at least to allow the bulbs to fully warm up before they reach their maximum brightness. I have a problem with my 451 that when I am running the low beams and I go to push the turn signal stalk to fire up the high beams, in the middle between low beam and high beam it turns out the low beams. It is just a split second but I worry that it may shortened the life of the HIDs. So when I switch to highs I usually flick that switch as quickly as I can. DCO