With all the Aliance stuff going on it is common to meet armies from different codexes. And units that do not have Fearless or ATSKNF, even if there are marines. Fearless on all models in the unit, come to think of it - isnt that particulary common.
There are more combos like Farseer with Shard of Anaris, Space Marine character in a unit etc.
Lots of Chaos Marines units also dont have Fearless, bar Cult troops and those who have daemon special rule. Cultist will enjoy dying from crazy ilusions particulary, since they have abysmal leadership value.
Our Eldar cousins will be glad also, besides Wraith units and Phoenix lord there are no Fearless in profiles. By the look of it invul saves can be taken against Tormentor Grenades effect, so Seer Council will be protected.
Inquisition types also dont have fearless. Dont remember if assassins have.
Necrons have zero Fearless across the board - particulary funny to kill robot with fear. Hope i catch someones attention at this point. )) Well, come to think of it - wraiths have fearless. They also have invul so it doesnt matter. Not a target.
We've mentioned Tau before - they are suceptible. Low LD value and no fearless.
With Tyranids ofc there is a need to remove synapse creatures first. Then they'le eat each other and then die from fear.
Imperial guard doesnt have much fearless also - but still tanks and apc doesnt have the need.
Orks lost their Fearless and gained new special rule - so they to can be affected.
I dont remember what do sister have with fearless.
That leaves loyal marines of all sorts to be unaffected. And they sometimes take Allies from the list above.
So really, taking tormentor grenades isnt tailoring.