Many rare stuff in game have similar drop chance, however there's just not enough necramechs for us to kill/farm. It takes 20-25mins to kill 6 mechs if skipping vaults, 1 in tier one, 2 in tier two and 3 in tier three. I have done around 40 T1-T3 iso vault runs with Nekros, yet I've gotten zero rare necramech mods. (not using Mod Drop Booster)
40*25mins is about 16.7 hours play time, kill counts: T1, 40 ; T2, 80 ; T3, 120.
I would suggest treat these as lucky jackpot for now and wait for future updates as other means to get the mods will likely be added. Hopefully it won't take too lon