That which melts your retina might not be that bright
The perceived brightness of a light source might mislead you to think that a light that doesn’t put out that much light is brighter than it is.
Allow me to illuminate this for you.
Consider a laser, which is a very focused columated beam. A laser that isn’t actually that bright can be enough to burn a hole in your retina. Class 2 lasers, the sort you shouldn’t stare at but are protected against damage because you will be able to blink in time, are limited to 1 mW of continuous power. By comparison a 20 mA 5mm LED is generally 40 mW of input and at least several mW of emitted continuous power and it doesn’t have the same danger of melting your retina.
Compared to incandescent bulbs, early white LEDs were incredibly bright if you shined them in your eye, but if you tried to use one as a flashlight, you’d discover that it was barely able to illuminate well enough to see with. The LED was emitting a lot of light from a really tiny chip and phosphor combination and so the little bit you were seeing was much more intense across that tiny section of your retina than filament of the incandescent bulb through a clear glass bulb and even less than a frosted glass bulb.
Furthermore, looking at a light from the sides also changes perception. A lot of LEDs come with a lens of some sort on the top and a cone-shaped reflector underneath the LED chip, which means that they tend to be at least some angle of a spot-light and not waste much energy around the side. Viewing the LED directly from the perfect angle is different from viewing it from the side. I tried a few bike lights that looked really bright in the store but when I tried to ride with them in the night, they only illuminated a tiny little spot of the pavement. I took a very dim view of this.
Thus, using even a relatively efficient diffuser can take things from really bright to barely visible and using a sub-optimal diffuser that’s absorbing light can do even worse. So a transparent LED package might seem like it has a clear advantage over one with a milky-white package, the milky-white one might be putting out more light.
Darkness might color your perception
Part of why red light doesn’t ruin your night vision is that the rod cells in your eyes that give you night vision aren’t sensitive to it, only the cone cells that sense color.
Color in low light or darkness tends to be skewed away from red so things start to go blue.
Your eye is not a linear sensor
Your eye is weighted towards picking out food and danger in a forest, so it’s got different areas that it is more or less sensitive to. The overall sensitivity peaks around green and falls off towards blue and red, which means that two LEDs of identical power but different spectra will tend to have a different level of brightness.
Also, a light at a given brightness can be more or less visible depending on where it is in a scene. Your ability to see colors towards the edge of your vision is really limited compared to the center of your eyes where it’s using the same cells that your eye uses for night vision.
Finally, your eyes have the ability to change light capture globally (by adjusting the iris of your eye) as well as locally by various chemical mechanisms that change your ability to see light over part of your vision. This means that something that’s really bright indoors might not be very bright outdoors and even less bright outdoors against the sky.
Figuring out spectra is not your eye’s primary responsibility.
Your M cones cover a range of light between 450nm and 630nm, where the peak is somewhere between 534–555 nm. A narrow spectrum of light right in the middle there is going to be perceptually brighter than a wider spectrum that’s putting out the same amount of light.
RGB LEDs and some of the lower-CRI phosphor formulations can take advantage of this, by providing enough white light that your eye registers it as white but without any “unnecessary” light in wavelengths your eye is less sensitive to and that works, except that everything looks weird.