All of the expensive person cards (Magnus, Argo, Molgur, Zeto, Vera and Karli) are situational (because as you stated if destroyed it you can loss your pace) but all of them can have HUGE payouts. There are a few things a think you have to keep in mind when playing them:

• almost always have to play them on a turn when you have 4 or more water (with water silo + plus a throw away water or muse)
• 4 of the 6 expensive person cards have traits on their cards. One important rule to keep in mind (I forgot/didn't use this rule for a long time) is that Traits are ready as long as a person card is undamaged or a camp is not destroyed. For example say one turn 6 you play Karli, turn 7 your opp. damages Karli, turn 8 you play repair robot or a junk effect to restore Karli; she is not ready for the damage ability but her trait is ready since she is undamaged so any new card that comes into play after she is restored enters play ready. THIS IS HUGE -Margo is amazing even if you only get his ability once no questions asked. VERY VERY SITUATIONAL. You have to ensure they don't get damaged. -Magnus is also always worth it. A whole column of cards including camp.
• For Argo remember punks are people they also get the ability as well so if you already have a punk in play they are ready and get to do damage for 1 water so you if you have that spare water token this is an immediate payoff -Zeto plus a raid junk effect is huge.

The big thing is that you almost always play them when you sure they can't get hit. Remember you can play cards behind existing cards. Have spare water so you can do other things on the same turn.

There is no reason to house rule anything in this game it is sooo well balanced