rnbw is nothing more than a pipe, where text goes in, and rainbows come out. Chances are, it may break something in the process of doing this.
rnbw's faster. Don't believe me? Here's a completely impartial and scientific benchmark:
$ time ls -al | ./rnbw
real 0m0.007s
user 0m0.003s
sys 0m0.004s
$ time ls -al | lolcat
real 0m0.070s
user 0m0.062s
sys 0m0.010s
rnbw won't stop dumping you colours if your tty doesn't appear to support them by default, has no dependencies and if printed out with a 14 point typeface would fit on an A4 page.