Is it dangerous to look at an L-type brown dwarf from too close? Are some L-type brown dwarves bright enough so that it would be dangerous to the eyes if one looked at them from habitable distance or closer? If not, are the weakest M-type stars undangerous to look at from habitable distance too? T-type brown dwarves are dim enough that there shouldn't be any danger, and Y-type ones are invisible.
A typical L-type brown dwarf is about 1200-2200 K in surface temperature and is about the size of Jupiter. Using the Stefan-Boltzmann law, we can deduce that the hottest brown dwarfs have a luminosity of$$\Big(\dfrac{2200}{5778}\Big)^4 \cdot \Big(\dfrac{1R_J}{1R_\odot}\Big)^2 = 0.00021224 L_\odot$$
According to Wikipedia, the dimmest apparent magnitude that causes pain to look at is $-25$. We can use the luminosity formula for apparent magnitude and distance calculate how far away an observer must be to experience this magnitude: $$0.00021224 = 0.0813   \text{ ly}^{-2}d^2\cdot10^{-0.4\cdot(-25)}$$
Solving the equation gives $d=0.03232 \text{ AU}$, or $67.62 $ times the brown dwarf's radius. Using this useful HZ calculator and plugging in $T_{eff}=2200$ and $L=0.00021224$, we get the distance must be between $0.015$ and $0.032$ AU, which is inside that limit. This range is inside the limit for $-25$ apparent magnitude, so no, looking at brown dwarfs at their habitable zone is not guaranteed to be safe.