Neim, 1 byte

𝐐

Simply checks that all elements are equal.

Without builtin, 4 bytes:

𝐮𝐣μ𝕃

Explanation:

𝐮      Calculate unique digits
 𝐣      Join list into an integer
   𝕃   Check that is is less than
  μ    Ten.

Try it!

Neim, 1 byte

𝐐

Simply checks that all elements are equal.

Without builtin, 2 bytes:

𝐮𝐥

Explanation:

𝐮     Calculate unique digits
 𝐥    Get the length

This works because only 1 is considered truthy in Neim, and everything else is falsy.

Alternatively, for 4 bytes:

𝐮𝐣μ𝕃

Explanation:

𝐮      Calculate unique digits
 𝐣      Join list into an integer
   𝕃   Check that is is less than
  μ    Ten.

Try it!

Neim, 1 byte

𝐐

Simply checks that all elements are equal.

Try it!

Neim, 1 byte

𝐐

Simply checks that all elements are equal.

Without builtin, 4 bytes:

𝐮𝐣μ𝕃

Explanation:

𝐮      Calculate unique digits
 𝐣      Join list into an integer
   𝕃   Check that is is less than
  μ    Ten.

Try it!