Why forms are a different problem
Everything else you dictate is sentences. A form is a series of short, unconnected fragments, and that removes the thing recognition depends on most.
Context is what lets a system choose between similar-sounding options. A sentence supplies plenty. A field containing one word supplies none, so the decision rests on sound alone, which is the weakest evidence available.
That single difference explains why dictation feels excellent in a message box and unreliable in a postcode field. Understand that one point and the app stops looking inconsistent from one field to the next.
Where dictation clearly wins
Long free-text fields. The "tell us about your enquiry" box, a delivery instruction, a message to a seller, a review. These are sentences, so all the usual advantages apply and typing them on a phone is miserable.
Search boxes with real words. Searching for a product, a recipe, a place, an idea. Speaking a phrase is far faster than typing it, and search engines are tolerant of small errors because they match text, they do not store it.
Repeated fields you fill in constantly. Anything you have added to your personal dictionary comes out reliably, which is what makes the difference for people filling in the same job details several times a day.
Anything where your hands are occupied. Standing in a queue, holding something, wearing gloves.
Where typing wins
Four kinds of field are quicker to type, and they have a shape in common: no sentence for the recogniser to lean on.
Email addresses. A symbol, a proper noun and a suffix that sounds like an ordinary word. Every part fails, and one wrong character means the form is useless. Type these, always.
Postcodes and reference numbers. Letters spoken alone are the hardest thing in recognition, because so many of them rhyme. A postcode is short enough to type in three seconds and expensive enough to get wrong.
Card details and anything financial. Aside from the accuracy problem, dictating a card number aloud is a poor idea in any room containing another person.
Passwords. Not a choice: a well-behaved keyboard withholds the microphone in password fields, and LocalType does.
Single short words. A first name, a city, a quantity. The overhead of tapping the microphone and checking the result exceeds the typing it replaced.
Addresses are the mixed case
An address is half prose and half code, which is why it sits awkwardly between the two lists above. The street name and the town are words, and they dictate well, particularly if you have added your own address to the dictionary. The house number and the postcode are strings, and they do not.
The practical approach is to split them. Dictate the words, type the numbers. That sounds fussy and it is faster than dictating the lot and then correcting two digits you have to spot first.
If you fill in the same address regularly, add the street and town to your vocabulary once and be done with it. Anything you add stays on the phone and is passed to the recogniser as background knowledge, exactly the help a lone place name needs.
Fields that change the keyboard
This one looks like a fault and is not. Android tells the keyboard what kind of field it is, and the keyboard responds: a number pad for a phone number, a layout with an @ for an email address, and different behaviour in a password box. That is deliberate and it is how a keyboard knows to be useful.
The consequence for dictation is that some fields will not offer the microphone at all, or will offer it in a field where the result is going to be wrong anyway. A number pad is a strong hint that the field wants digits, and an even stronger hint that speaking is the wrong tool.
Treat the layout as advice. If the keyboard has decided you need numbers, it is probably right.
Browsers and web forms
Most form-filling happens in a browser rather than an app, and this is where a keyboard has a structural advantage over a dictation app.
A dictation app produces text somewhere else, which then has to be copied into a field in a page. On a phone, moving between a browser and another app while keeping your place in a long form is genuinely unpleasant, and it is why people give up and thumb-type.
A keyboard is simply there, in every field, in every page, in whichever browser you use. Tap the field, tap the microphone, speak, move to the next field. Nothing is copied and nothing loses your place.
Checking before you submit
Forms deserve more checking than messages and less than they usually get.
The error that matters here is not a clumsy sentence. It is a wrong digit in a field that will be acted on: a delivery address, a phone number, a date. Those are silent failures, since a wrong number looks exactly as plausible as a right one, and the consequence arrives days later.
The routine is short. Scan the fields containing numbers, check the email address by reading it character by character if you dictated it at all, and ignore the prose fields. Those are forgiving.
Autofill is still faster than either
Neither dictation nor typing is the fastest answer to this problem, and the reason is a feature you already have. For the fields you fill in constantly, name, address, email, card details, Android's own autofill is faster than dictation and faster than typing, and it does not make mistakes. If a form offers to complete itself, let it.
Dictation is for the fields autofill cannot know: the message to the seller, the delivery instruction, the search phrase, the description of what went wrong.
Framed that way the division is clean. Autofill for what is already stored, dictation for what you are composing now, typing for the short strings in between. Most people use exactly one of those three for everything and wonder why forms are tedious.
What stays the same
Recognition happens on the handset from a model in storage no other app can open, so a form filled in on a train works the same as one filled in at home. Internet access is used for exactly one thing, collecting the model you chose.
No account is involved, advertising is absent, no behavioural tracking runs inside the product, and nothing keeps an archive of what you dictated. Android's cloud backup is denied the app's data deliberately.
The password behaviour matters especially for forms, since that is where login fields live: the microphone key is withheld there, so a password cannot be dictated by accident into a page.
Look at the field first
Look at the field before you decide how to fill it.
Prose field, speak. Number or code, type. Something you fill in every week, let autofill do it. That decision takes half a second and it is the difference between forms being noticeably faster and forms being a series of small frustrations that make you conclude dictation is unreliable.
It is unreliable, in exactly the places listed above, and completely dependable in the rest.