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.
I see it going that way too, which means you're incentivised to use them to scout by design and disincentivised to use them to scout by practice.
It's not bad design, per se, it just feels...a bit flat, a bit clunky, a bit meh
There's something fundamentally fun about the fact that even in tier4 with god like arcane powers, most of the time the wizard Has To Go To The Place To Do The Thing. Maybe that's why Simulacrum inspires the same feeling, that it's powerful but is it fun?
I find it an excellent scout as long as you were not planning on being quiet anyway. Can even cast some kind of crowd control spell through it before ever being in danger, if the situation allows.
True, but I was thinking more in a dungeon, where a simple door, can block them (or make them take a completely different, unique route. Also note that invisible creatures still need to make Stealth checks, usually more than one.
Invisible creatures are heavily obscured, so out of hearing range they automatically pass stealth checks. And if they're magically flying they probably aren't making any noise either. Maybe smell???
If I was more experienced as a DM I'd have required the familiar to wear something that has a risk of causing noise, when the player made the character.
Since I'm a fairly inexpirenced DM, we're using a premade campaign, without many traditional dungeons-crawl type dungeons (I personally dislike dungeons-crawl dungeons), and I'm afraid to substantially change the ones that came with the it.
I'm hoping that when I make my own campaign, I can work these problems out since I'd feel more in control of the environment.
For reference Invisibility would make you invisible, but you still can be detected by sound, smells, tracks, etc. it’s not an instant win, for stealth. As such, you get advantage on Stealth checks.
Spell aside, have you considered other game systems? Something that doesn’t use dungeons? There are many around, some are more RP oriented, some more base building/strategy oriented, and so on. I say this because a dungeon crawl is a classic experience for D&D, I mean it’s right in the name.
I was a bit unclear about the rules honestly, they seem ambiguous. It says that for the purpose of hiding, invisible creatures are heavily obscured, and any check requiring sight automatically fails. Perception doesn't require sight, but what other sense would it use out of earshot and smellshot? How far is earshot? I imagine it's gotta be pretty close for a naked creature using silent magic to fly. What would a failed stealth check even mean (how would they be detected), if they're downwind, invisible, flying, and their flight magic is silent?
It also mentions attack rolls, but I don't see it mention stealth checks anywhere. But tbf I'm just going off roll20 - I find looking up special cases in the DMG painful (I have a hard copy, not searchable on roll20).
Wrt game types, were currently in the middle of a huge campaign, and so I can't change it right now. But I do really want to try Blades in the Dark (Forged in the Dark system). But there are plenty of D&D campaigns that don't have classic labyrinth style dungeons, even by WOTC. I compare, for example The Lost Citidel vs The Storm King's Thunder; the latter has very few labyrinthian dungeons, while the former is entirely labyrinths. They're dungeons, but of a different style, and I think that's ok.