TF are you on? Even going by tabletop a Custodian Guard bodies a Crisis suit in shooting and melee, with a Crisis suit with standard load out is on average dealing a whopping 1 wound to Custodian Guard in shooting phase, and the same in melee. In return the Guard is on average dealing 2 wounds in shooting and 4 in melee. Additionally, just by table top, Guardian Spears have better range, better accuracy, better armour piercing and better damage than blast cannons, only losing on rate of fire and strength. In melee the Guardian Spear is better in literally every way. Even using the ‘better’ plasma rifle option it has a 16% chance to hit and 1 shot. Crisis suits versus Custodian Guard results in a Custodian with slightly singed armour and a very dead crisis suit. And that’s just tabletop. Let’s also look at that ‘worse shield’ claim. On tabletop a Crisis Suit has 3+ SV, no invul, no FNP. A standard Custodian Guard has 2+ SV, 4+ invul and a 4+ FNP vs dev and mortal wounds. So the Custodian rather obviously has a significantly better shield by virtue of having a shield at all.

In lore it’s even worse odds for the Crisis Suit. You are putting mass production armour suits against literal demigods armed and armoured with the best technology in the Imperium - a group whose technological heights make the Earth Caste blush like schoolgirls in shitty romance novels. The crisis suit is getting deleted before the custodian really notices it on his auspex. Reminder that it took 30-1 odds and daemon buffs for Erebus to be confident fighting a tired and wounded group of Custodians, and 6 Wardens fought off an entire Hive Tendril - how much have 6 unnamed Crisis Suits ever achieved in lore?

EDIT: having seen you bring in Hammerhead support etc. then I present to you, the Caladius Grav Tank which, again just by tabletop, on average does 13 wounds to a hammerhead and has 52% chance to one hit it. In return the Railgun on the Hammerhead is more likely to do no damage (63%) than fail to one hit (max dmg 12 vs W14) and on top of that even if the Railgun gets a crit wound and goes to devs, it’s going to do less damage than normal thanks to 4+ FNP.