I also find other mercs I have no favoured card and it’s all map dependent. Like bushwhacker for example. I don’t use engineer often anymore but if I do I literally have 3 cards I use for him depending on what map it is. On bridge for instance I find the turret more handy than ever, thus I prefer a turret based loadout like my bamboo gold C41. On chapel or any map incredibly succeptible to airstrikes on the EV, I prefer to have an airstrike safe card like my CW or cobalt C82 (<3 the guardian angel perk). Finally on enclosed maps like underground where you’re more likely to encounter nader nades / fragger nades / any projectile based damage more than anything, I prefer to use my alienware SM41 basically just for the unshakeable perk.
Bushwhacker is just one example any way. With other mercs I find similar things. I don’t really have just one card for all (other than Arty, that card is just too good for all maps really)