My neck gets sore and goes into spasms easily from too much weight or stress (permanently damaged C2 vertebrae from old injury). The way I handle lights and even a video camera on the helmet is to mount them top center whenever possible.
Also, I ride fairly upright -- either a comfort hybrid with flat bars slightly above saddle height, or a rigid fork hardtail mountain bike with slight riser bars roughly at saddle height. If I used a road bike or other bike with drop bars I might notice more weight on my helmet bearing on my neck.
For more than I year I've ridden every ride with a Vivo-Bike Illuminati headlight (approx 35-50 gm, maybe less) mounted top center and slightly to the right; and a Blackburn 2'Fer (18 gm, updated version of the Flea with standard micro-USB port charging) on the rear. I don't even notice the weight added to my Bell Solar helmet (about 350 gm, IIRC).
Occasionally I mount my Ion Speed Pro video camera on top center. I homebrewed a mount to keep it as low as possible -- I don't use the tall ballhead or articulating mounts. Those aren't really necessary with the Ion since it has a nearly fisheye lens and precise aiming isn't necessary. Supposedly it weighs only 4.5 oz (128 gm) but feels heavier -- probably the additional weight of the mount and height affecting the balance. Usually I don't mind but occasionally I prefer the camera on the bike. It's just convenient to have the camera on the helmet when switching between bikes, and it does record wherever I look -- handy since I have only the one camera.
A few times I've even rigged a USB battery to the helmet to extend the recording time of the Ion Speed Pro beyond the usual 90 minutes. It'll run for hours or until the 32 GB card is filled. I notice the weight and don't do that often. If I did I'd use a smaller cylindrical battery and use Velcro to keep it from moving.
No real problems with those methods. But any weight on a visor at the front of my helmet would probably tire my neck quickly. So I keep the weight top center as much as possible.
If I add another camera it'll be a Mobius, which weighs only 40 gm. Perfect for the helmet.