That first one is worth setting up before anything else. Somebody who fills in your contact form is at their highest point of interest and their lowest point of certainty, and a message confirming you received it, saying when you will reply, and giving them something useful in the meantime converts that anxiety into confidence. It also buys you time, because a stated response window is not the same as silence.
The mistake is treating it as a marketing opportunity. An automatic reply that opens with promotional material reads as the machine it is. Keep it short, human, and specific: what you received, when you will respond, and a phone number if it is urgent. If you can include one genuinely useful thing, such as a link that answers the question people usually ask next, it earns its place.
The second one worth having is a welcome message for anybody joining your list, because that is the moment they are most likely to read anything you send. It should say what they will receive and how often, which sets an expectation that reduces unsubscribes later, and it should deliver whatever you promised at signup immediately rather than in a future send.
Beyond those two, resist building sequences until you have enough subscribers for the effort to return anything. A five message onboarding series for a list of forty people is a project that produces almost nothing, and the same hours spent talking to customers would produce more.
Test what you build by sending it to yourself and reading it on a phone. Automatic messages are written in a desktop editor and read on a small screen, and the difference exposes problems that are invisible while writing: a greeting that mangles a name when the field is empty, a signature longer than the message, a link that is not obviously tappable. Check what happens when somebody submits the form with the name field left blank, because a message beginning with a stray comma is the version a real person will receive.
Review what is currently sending every few months, since automatic messages outlive the circumstances that produced them. An out of office still active weeks later, a form response referencing a service you no longer offer, or a link that has broken. These fail silently and the first indication is usually a confused reply from somebody who received one.