It's more compact than UTF-8 because fewer bits are used for the encoding itself. There are 24 bits, and only 3 bits are used as part of the encoding, with the other 21 bits representing the code-point. (Precisely the number we need to represent all Unicode).

In UTF-8, a 3-byte encoding uses 8-bits as part of the encoding, a full byte worth of bits for the encoding itself, leaving only 16-bits for the codepoint. If you need higher code-points you need to use 4 bytes, where 11 bytes are the encoding and 21 bytes are the codepoint.

So UTF-8 is space efficient for ASCII, but ~1/3 of the bits are used for the encoding in for non-ASCII, versus a fixed 1/8 of the bits used for the encoding above for all 1-3 bytes. The above has a fixed 12.5% space overhead over raw codepoints. UTF-8 has 12.5% only for ASCII, and ~33% overhead for everything else.

Although it is not self-synchronizing like UTF-8, you can synchronize a reliable stream by holding a buffer of the previous byte. If the previous byte's value is >=0x80, the current byte is part of the same character. If it's <0x80, the current byte is the start of a new character, so it's still possible to do substring matching etc, fairly efficiently but slightly less efficiently than UTF-8. It makes it suitable for file storage, but not ideal for transmission.

That said, most sane protocols will prefix a string with a length (in bytes), so self-synchronization is not always an issue.