This is a bit like suggesting you've never been in a car crash, so seat belts must not be worth considering.
Do you feel that beyond the obvious and documented work in setting them up, there are disadvantages to using SSH certificates?
However, if you do not need the extra features provided by certificates, using SSH-generated keys is strictly equivalent with using certificates and it requires less work.
TOFU is neither necessary nor recommended, it is just a convenience feature, to be used when security may be lax.
The secure way to use SSH is to never use TOFU but to pair the user and the server by copying the public keys between the 2 computers through a secure channel, e.g. either by using a USB memory or by sending the public keys through already existing authenticated encrypted links that pass through other computers. (Such a link may be a HTTPS download link.)
When using certificates, a completely identical procedure must be used. After certificates are generated, like also after SSH keys are generated, the certificates must be copied to the client computer and the server computer through secure channels.
Just to make it clear: this does not mean that it is fine to blindly accept the message on first use.
The "secure way" implies copying the server's public key as well, which people generally don't do, right? Which is equivalent to verifying the fingerprint shown with the TOFU message, correct?
The server public key must be copied into "known_hosts" on the client, while the client public key must be copied into "authorized_keys" on the server.
When this is the procedure that is always followed, any message shown by SSH about an unknown host means that the connection must be aborted, because the identity of the server is unknown.
You cannot truly verify the "fingerprint" displayed by SSH, unless you simultaneously have access to another computer, where you have a copy of the fingerprint. What is usually meant by "verifying" is that you remember a few digits of the fingerprint, and those match.
You could have copied the fingerprint from the server, to be able to truly verify it, but that does not make sense, because in that case you should have copied the entire key, not just the fingerprint, and you should have installed it in the client.
When you use only authentication with digital signatures, it does not make sense to use any other procedure, because you must make at least one of the two copies anyway, so when copying the client key to the server you can take the server key, to carry it back to the client.
The TOFU method is meant to be used together with password-based authentication, in less secure applications, where no physical access to the server is required for setting up SSH on the client.
By "less secure" I mean for example applications equivalent to HTTPS, where the client is not really authenticated, e.g. when providing a public password allowing read-only access to an SSH server through Internet.
Sure, I just wanted to make sure that nobody would understand "TOFU is neither necessary nor recommended, just ignore that message and say "yes" when it appears".
Both ends need to be sure of who is on the other end, and there are different ways to achieve that. The way SSH works is that if you haven't copied the server public key locally, it will explicitly ask you to verify it the first time.
I am not sure that "SSH does TOFU". SSH asks you to verify. The human who YOLOs it and approves it without checking is the one doing TOFU, and this is not really secure.
That is not the case, and is a major advantage of certificates.
When an authenticated connection between a client and a server (or any other 2 computers) is established, before the connection can be initiated both ends must already have a piece of information that has been copied to them through a secure channel, otherwise the authentication is impossible.
The 2 pieces of information may allow direct reciprocal authentication, i.e. the client may have the server certificate and the server may have the client certificate.
It is also possible to have indirect authentication, e.g. the server sends the server certificate to the client and the client sends the client certificate to the server during the initiation of the connection.
However, for indirect authentication, both the client and the server must had been previously paired with a server hosting the certification authority.
This means that the root certificate of the CA must have been transferred through secure channels to the client and to the server, and also that the client certificate and the server certificate must have been transferred through secure channels to the CA for signing and then back to the client and the server.
So setting up the computers for indirect authentication through certificates requires significantly more transfers through secure channels than setting them up for direct authentication.
The advantage of indirect authentication appears in an organization with a great number of computers, where indirect authentication reduces the number of secure pairings that must be done from N*(N-1)/2 to N.
However this has nothing to do with certificates. Indirect authentication can be done without certificates, like in Kerberos, or like in SSH if you have a management computer that you pair by copying the public_keys with every computer in the network. Later, whenever you want to enable some client to connect to a SSH server you use the management computer to copy securely the corresponding public_keys between the server and the client.
So no, there is no advantage of certificates related to reducing the number of transfers that must be done through secure side channels, like copying through a portable memory.
Certificates provide other features, like temporary validity and revocation, which can be useful in an enterprise context.
If you use SSH certificates, then you have to put the SSH CA cert on each server once, and you have to give signed certs back to each client once each time you sign a cert (which can be once per client or once per connection or anywhere in between).
You do not have to copy anything new to each server when you sign new client certs. Likewise if you sign host keys, you don't have to copy a new host key to your client when you add a new server.
Sure, you could get the same kind of trust relationships with other tools. But SSH certificates provide them and are an advantage of SSH certificates over traditional SSH public key auth.
It honestly feels like you're just dropping this thread into an LLM and asking it to write a disagreeing response.
Aside: We need a "safely dumb" storage plug standard. Right now the flexibility of USB is a double-edged sword, any USB device could potentially be malicious, waiting for the right moment to send keystrokes as a keyboard etc.
Or use the same plug for everything, but support a "passive storage only" that can be enforced by a special adapter or cable.
When you’re getting things from strangers, use any of the myriad ways to send files over a network, where HTTP / your browser are the safe dumb plug.
A USB stick that can secretly remember any file that looks like a key file or certificate would be very easy.
As for your ISP I think you should never rely on TOFU over the public internet. If you really don't want to do ssh certs it's easy enough to make the host key available securely via https.