After reading around as a new player, I've discovered how powerful the Find Familiar spell can be for creative players—albeit it's possible I've misunderstood the leeway Find Familiar affords.

For example, one thing I read is that certain familiar forms like the Owl, Bat, or Weasel can detect stealthed enemies for you. It can also be used to scout ahead and prevent ambushes.

I then compared this to the feat Alert:

Alert (PHB 165)

Always on the lookout for danger, you gain the following benefits:

• You gain a +5 bonus to initiative.


• You can't be surprised while you are conscious.


• Other creatures don't gain advantage on attack rolls against you as a result of being hidden from you.

Alert also "prevents" ambushes, at least the surprise element, although it's inferior in that your team presumably still isn't well positioned.

Alert also prevents the advantage from stealth. Although again, one might argue it's inferior in that actually being aware preemptively of a stealthed target's presence would be better.

Barring the +5 initiative, this makes it seem like Find Familiar is a reasonable if not better substitute for the Alert feat's benefits.

Question

In what ways are the last 2 bullets from Alert not obviated by Find Familiar (or in what ways have I misjudged Find Familiar)?

Alert is always active and infallible. Familiars are not.

Surprise is not always about hiding.

The party can surprise a foe or be surprised by an encounter without the enemies being hidden. E.g. a ball or gala where some of the guests are disguised assassins. They're not hiding. They're disguised. When their coordinated attack is launched, the character with alert will not be surprised, while a familiar will be just as surprised as a character.

Familiar does not prevent hiding. Alert mitigates the advantage regardless.

A sufficiently stealthy enemy is likely able to sneak up on you in the presence of your familiar as easily as without it. The familiar is an additional set of eyes and is not everywhere at once. E.g. is your familiar scouting ahead for an ambush, or watching one of your flanks, or tailing the party to guard the rear?

Familiars don't mitigate attack bonus from unseeable enemies. Alert does.

Familiars do not grant the ability to see invisible enemies nor the ability to see in darkness beyond 60'. Attacks from beyond the ability of the character to see will still have advantage despite a familiar. Alert mitigates attacker advantage even when the attacker is unseen.

The errata for the feat reads:

Alert (p. 165). The third benefit now reads, “Other creatures don’t gain advantage on attack rolls against you as a result of being unseen by you.”

Familiars can get killed. The alert feat cannot be killed.

Sending a weak creature ahead even stealthily or invisible is likely to get it killed in a hostile environment. It costs an hour and 10gp worth of specific materials to re-obtain a familiar. Alert cannot be inactivated and does not require casting.

Familiars are not guaranteed to perceive dangers.

But as you noted, some of the familiars have nice perception scores/passive scores. Tatical not in re scouting: the familiar may be detected by a hidden enemy while it is scouting, perhaps betraying the party's presence and giving the enemy a chance to strike first.

Owl, Bat, or Weasel can detect stealthed enemies for you. It can also be used to scout ahead and prevent ambushes.

Yeah, they are great scouts in my experience.

But...if the enemy rolls higher on their Stealth check than the passive perception score, or the perception check of the familiar, the enemy may still achieve surprise/stealth. The bugbear is a nice example.

Skills Stealth +6, Survival +2 / Senses darkvision 60 ft., passive
Perception 10 / Brute. A melee weapon deals one extra die of its
damage when the bugbear hits with it (included in the attack).

Surprise Attack. If the bugbear surprises a creature and hits it with
an attack during the first round of combat, the target takes an extra
7 (2d6) damage from the attack. (MM, bugbear).

Alert is always on

Bullet two is extremely valuable for preventing surprise. Surprise can massively swing a combat - particularly if the enemy has numerous attacks, crowd control spells, breath weapons, or numbers.

Advantage on an attack roll increases the chance of a critical hit on you from about 5% to about 10%. Particularly at low levels, critical hits can be encounter ending for a given PC. Reducing the chance of that is nice when you are a squishy caster.

While I personally like having the boost in initiative, I've seen arguments that the swinginess of the d20 roll mitigates that benefit somewhat - and you may not necessarily want to go first.

How much sneaking around and ambushing is in the campaign?

That will inform which may serve you better.

Familiars have a lot of utility, and also have the advantage of not costing an ASI feat. Also, in a game without feats (they are an optional/variant rule) they may not even be available. Find familiar is a favorite spell among casters for a lot of good reasons, but any class, spell caster or not, can take the Alert feat.