Poison and Sniper weapons have a reduced effect against GMCs, only wounding on 6's against them.

I feel this is a horrible mistake.

GMCs are very tough. Many of them are flat out immune to the small arms fire that makes up the bulk of most armies. Even light tank killers such as autocannons struggle to meaningfully wound a wraith knight, and there could be 5 of those wandering around the field in a 1850 point game.

Even those wounds that do get through its armour run into a 5+ feel no pain, a constant for all GMCs. Given cover shenanigans, a GMC can easily have a 4+ cover 5+ feel no pain, on top of 7+ toughness and 5+ wounds.

Most armies are going to have a LOT of trouble killing that, even discounting tricks like making them invisible or giving them shrouded via psyker. Not everyone can spam ranged Strength D or undermounted Grav weapons.

This is why sniper and poison weapons should not be nerfed against GMCs. They're available to almost every army in one form or another, are a hard counter to GMCs at the cost of being ineffective against cannon fodder and vehicles.

This hard counter is desperately needed in the current meta, where Wraithknights are considered severely undercosted for their durability. Even with the hard counter a Wraithknight's still got a 3+ 5+FNP between it and poison, thus hardly rendering them helpless against such foes, so they hardly become useless, and with the exception of Dark Eldar no-one can spam mass ranged poison.

That's my thoughts on the matter. If you want to convince me otherwise, go right ahead.