Solar filters limit not only visible light but also ultraviolet and infrared light from the sun.
The primary concern with using ND filters instead of solar filters is not the difference in the amount of visible light reaching the camera. It is in the amount of invisible UV light and IR light reaching the camera when standard ND filters are used.
Infrared is particularly dangerous as your retinas have no pain receptors or other nerves that detect heat. You can literally 'cook' your retinas by looking at the sun through a high power lens and not feel the first twinge of pain!
So the absolute first rule is: Never look directly at the sun through the viewfinder unless a proper SOLAR filter is in front of the lens.
As long as you don't keep the camera pointed at the sun continuously, the camera will probably be OK if you use a combination of ND filters that add up to 14 stops. Keep in mind that in Live view the shutter stays open and the sensor is continuously exposed to whatever light is projected by the lens. In viewfinder mode the sensor is protected by the closed shutter, but the secondary mirror reflects a portion of the light from the image projected by the lens down into the autofocus array.
After you do it you might always wonder if you caused any harm to your camera's sensor or PDAF sensor.
I photographed the transit of Venus back in 2012 with my brand new Canon EOS 7D, a 70-200mm f/2.8 lens set at 130mm or less (to reduce the concentration of the sun's energy), and a piece of welder's glass taped to the front of the lens. The glass wasn't quite large enough to cover the entire front element and there were narrow slits at the top and bottom that let unfiltered light fall on the edge of the lens. The entire event was within a couple hours of sunset, so the sun was never higher than about 30° above the horizon. At lower angles in the sky the energy from the sun is reduced as it has to pass through more atmosphere to reach the ground.
Welder's glass does protect from infrared and ultraviolet as some forms of welding produce IR and UV in addition to visible light. I used Live view to manually focus and frame the tripod mounted rig, then switched back to viewfinder to take the shots via a cable release without looking through the viewfinder. I could see the preview images on the camera's LCD immediately after each shot was taken to know when the camera needed to be re-aimed as the sun moved down through the field of view. With only a 130mm focal length, even on an APS-C camera framing was not critical as I was planning to crop significantly as the sun was only about 10% the height of the shorter side of the image.
When I started shooting in lower light with the 7D I always thought it was way too noisy at high ISO with a LOT of shot noise, particularly in the red channel. It was more than my 50D by a pretty large margin. I'll always wonder if shooting that transit of Venus had anything to do with it.