In code which uses tabs one logical indentation level is always one tab. You can show tab with different width in you editor, but the source code is consistent.

If spaces are used you never know how many spaces is one logical level. There is no agreement - some people prefer 2 spaces, some 4 and may be somebody uses 8 or 6.

Even worse - I sometimes see when in a single source file some parts use 2 spaces for one logical indent and some - 4 spaces.