Cloaks for backstab (and other melee specials) don't give it for units with no equippable weapons

[Discontinuum]

Cloaks for backstab/infection/etc don't give the special if the unit has no equippable weapons (like Vampire Bat or 3-4 lvl Ghoul advancements).

[Dugy]

That was a feature since the time I first implemented the weapons logic nine years ago. The modifiers are added to all weapon types, so it affects only attacks that can be considered weapon types. This also has the consequence that ranged attacks with melee weapons (like spears) are affected by melee bonuses.

Of course, if you really insist, it can be changed, it's not a limitation, it just makes the internal logic simpler.