What drives a poor reputation for the Rogue class?

[Dr.Samurai]

I get where you're coming from, but even on its own merits without considering other subclasses the ability just isn't good. Even with the "unfailing" language protecting you, the identities aren't infallible - picking an identity that belongs to someone else is an automatic fail, and the DM gets to determine what an "obvious reason" to disbelieve your identity is, so it's not at all hard for that 7 days of prep time you can't even be sure of getting in a given campaign (let alone leading into the specific session you need it) to be flushed down the drain in the space of a single interaction. And for that dubious privilege, you gave up an entire subclass feature. The whole thing is just poorly conceived; they tried to improve it in the UA by removing the prep time, but succeeded only in making it redundant. They need to go back to the lab and redesign the subclass from the ground up.

I don't find the "an identity that belongs to somebody else" argument compelling unless your DM is the type of person to say "So sorry, but in your 7 days of work and with all the money you spent, you failed to uncover that Mr. Sneaky Infiltrator the Third is actually a real life person and so your efforts totally fail.

"Obvious reasons" are in fact up to the DM. But what we know for sure is that there is no reflexive contested roll. People just assume you are who you say you are, which is quite different from a regular Disguise check. Drawing on DM fiat here is going to open a huge can of worms with skill checks in general, and with spell resolution, so let's agree that things depend on the DM and move on from this argument.

The point isn't that campaigns will have enemy mind-reading or divinations all over the place though. The point is that in the campaigns that don't have those counters, you don't need a dedicated Infiltration subclass feature anyway, because without those things you can get by with a bog standard disguise kit and Deception Proficiency/Expertise anyway.

Again though, you're not just making a disguise. You're creating a person that other people believe actually exists. This is way more than fooling someone for a scene and moving on. This can lead to a tremendous amount of information gathering, and facilitate other party member abilities, such as obtaining items for the wizard to scry with, letting the wizard know what day and time to scry on the person so it's not just a crap shoot that you're going to scry on anything useful.

2014's false identity is foolproof until it isn't (and even when it is, what benefit that specifically gets you is vague at best)

It is essentially foolproof until you do something obvious to potentially blow your cover, then I'd imagine it's a series of checks. But having an all but foolproof identity can go a long way before something like that might happen.

A failure of imagination on what you can do with false identities is not "vague at best".

Assassin needs something different.

Like with most martial subclasses, they just need something more. But it would be in addition to this ability, which does actually provide benefits to the assassin.

Even if this is true it causes a different problem. If there is a narrative ceiling on what a Disguise Kit can do that only the Assassin is able to exceed, now you're limiting what kinds of infiltration stories get to be told when there isn't an Assassin in the party.

Not at all. As I mentioned, you'd need to carouse to make contacts, call on those favors to help spin the fiction that this identity is in fact a real person, forge documents to support the identity, and there's probably going to be social interactions above and beyond the carousing to do this. Then make the disguise, then deal with the skill checks when you execute and use the identity. And gold spent. As an example, it costs 250gp to carouse with the upper class.

The assassin just gets to do this and do it better as a feature and people just believe it until you give them an obvious reason not to.

there's still the issue of the Assassin's stories needing 7 days of prep time to set up.

Many games have downtime. An assassin with downtime can craft as many identities as they have time and gold for, after which they can assume that identity in later adventures that might not allow the time for new identities to be crafted.

I'll be quite happy to see the back of this particular ability in a few months time.

I can see it now.

"Your years of being a killer have gifted you with supernatural powers that make you better at your craft. At level 3 you can summon spectral knives with spectral poison on them. A successful attack deals 1d6+your class level in spectral poison damage, and the target is poisoned... spectrally. At level 9, you have honed your craft to near perfection. You can cast Disguise Self, Darkness, Darkvision, Charm Person, Knock, Levitate, and Shadow Blade each a number of times per day equal to your proficiency bonus. At level 13, your reputation proceeds you. You can cast the Fear spell 1/day. At level 17, your touch is literal death. As an action, you can touch a creature and deal a number of necrotic damage equal to three rolls of your Sneak Attack damage."

1) As mentioned, you're not immune - you have no way of knowing with absolute certainty that you chose a persona that doesn't belong to someone else in the world, unless you're secretly the DM, nor what actions/inactions might trigger the "obvious reason" failstate.

This is a supremely weak argument, but I do find it hilarious to imagine the assassin with his false identity walking into a room and bumping into the real life version of his exact identity that he didn't even know existed. Comedy gold.

2) Even when those aren't a concern, a numerically high check (20+) functionally does the same thing by letting you defeat passive defenses, because they are fixed.

I think the only time Passive skills are mentioned is with Perception vs Stealth. It's not obvious to me that someone's Disguise check is vs a Passive Perception/Investigation, or that their Deception checks to keep the bit going are vs Passive Insight checks.

Secondly, the DM decides what checks are necessary and if they are possible. Someone that is an established identity in a merchant guild, thieves' guild, religious cult, high society, etc. may go much further in conversation without a check being required than a person that has a nice disguise but that nobody actually recognizes as an already known entity with ties to their organization/group.

So asking for the time and location of a meeting and to join that meeting may not even be possible to succeed at just because you have a disguise. But if you have a false identity that everyone believes, maybe they will tell you just for asking. Or maybe it still requires some finessing and the DM asks for a roll. But it might be possible whereas just having a costume doesn't making it possible.