-x '1 2 3' -y 4 5
[download]

would have to be passed as

--extra-ffmpeg-param='-x '\''1 2 3'\'' -y 4 5'
[download]

You'd need nested quoting, which is very tricky.

This can probably be mitigated by mixing quotes.

--extra-ffmpeg-param='-x "1 2 3" -y 4 5'
[download]

And this isn't just tricky to build manually; this is also tricky to build programmatically from the shell. Like, what if you wanted to wanted to do the equivalent of ffmpeg -x "$x"? It would look something like

# to_shell_lit() - Creates a shell literal
# Usage:  printf '%s\n' "$( to_shell_lit "..." "..." "..." )"
to_shell_lit() {
    local prefix=''
    local p
    for p in "$@" ; do
        printf "$prefix"\'
        printf %s "$p" | sed "s/'/'\\\\''/g"
        printf \'
        prefix=' '
    done
}

--extra-ffmpeg-param="$( to_shell_lit -x "$x" )"
[download]

(Source)

This is part of the reason I suggested using multiple args.

-X '-x 1 2 3' -X '-y 4'
[download]

-X "-x $x" -X '-y 4'
[download]

-X 'x=1 2 3' -X 'y=4'
[download]

-X "x=$x" -X 'y=4'
[download]