Dungeons and Dragons

As a GM I could work around the familiar exploring. What's the range of control? What's it's passive perception, or stealth? What's the CR of perception checks for traps, or stealth rolls for enemies? Does the familiar have dark vision? Is it heavy or tall enough to trigger a trap?

As long as i know the abilities of the PCs, including familiars, I can balance the encounter. That's part of the job, really. Make sure the encounter is balanced and fun* for the players.

Even if the familiar's perception is high enough to find traps, that's not an issue if you use the PC's greatest enemy: doors. "As Mister Snugglebutt turns the corner they see a closed door barring their path. (On a successful perception check) A few feet up the path there is a small fissure. It leads to a narrow passage that winds down further into the caverns. A light appears at the end of the passage, and the sound of movement can be heard in the distance. As Mr Snugglebutt approaches…what was the range on control again? (Meanwhile behind the door is a trap laden corridor that the familiar bypassed entirely, but the party must use)

tl;dr: give me a familiar and I'll give you The Adventure of the Alerted Guards: How My Cat Just Turned This Into a Prison Break Story.

*for a given value of fun

Pact of the chain warlock player here. It's hard to make it fun for everyone.

As a player, my fantasy is having my invisible or spider shaped familiar go in and get us intelligence so we can get the jump on stuff. I want to know there's four bandits eating lunch and one on watch so we can make an informed plan.

It is extremely not fun when the DM has characters stomp on my rat specifically. Or when they just happen to hear an invisible creature and just happen to decide to shoot it. Nevermind that there are rats everywhere, and an invisible flying creature is just as likely to be something Really Dangerous as a familiar. It's annoying metagaming.

The dnd rules are also kind of bad about invisibility. Shooting a flying invisible creature that's out of range 300' away is the same disadvantage as shooting a visible flying creature that's 300' away, or an invisible one that's 30' away. It is extremely annoying when the bandit decides he hears something funny up above so he decides to shoot it. The tiny invisible thing. And then rolls two tens and kills it.

When the party has someone with something like pact of the chain, the dm needs to move away from the surprise as tension. You can generate tension in many other ways. Time sensitivity. Or just forcing the players to make a hard decision. Cross the troll bridge or go under through the crocodile River. Knowing the troll is there ahead of time isn't that big a deal.

The player needs to not try to take all the spotlight. Don't try to pull a "I poison all of their food with my familiar". Dm needs to speed up the scouting part.

Also hidden traps kind of suck. Familiar won't find most of them, but probably don't use them. Stuff the players make decisions about are more interesting

I think it depends. Entities in a potentially hostile area can still detect the Familiar, or Manifested Mind. I think it’s unlikely they’ll make it past 1/2 rooms, never mind an entire dungeon undetected. The entities are likely seeing this as a hostile action, and now know that there is someone nearby. They will prepare, and potentially make the situation worse for the PCs.

Edit: Should also note that abilities like Manifest Mind is VERY up to interpretation. It has no stat block, and is very limited. It can hover just above the ground, not fly, and cannot pass through objects. In other words it cannot do skill checks, cannot pass a closed door, and because it sheds light, it’s a beacon for creatures to detect.

Adding into this is tracking of time for the moving of the familiar / manifest mind. If a DM never tracks time for scouting then some of the tension is lost. Look at the dungeon movement rates for the 10 minute dungeon turn (I used combat movement x4 for standard 10 minute turn. This is standardize against based on some older rule sets but the DMG has a similar formula).

So by tracking this spell durations will expire, enemies will move throughout the dungeon or wandering monsters will happen. Without tracking time a lot of important decisions are missed.

I think it is still fun. Just a different kind of fun. It shifts to a taktical game with lots of interesting strategic discussions instead of an exploration game.

Usually when I get annoyed that someone's familiar is taking all the intrigue out of a dungeon, it gets smashed and all of it's 1hp vanishes.