Nuclear weapons are kinda on the pricy side and they won't affect the war index (meaning they won't help you win a war). They are more or less for deterrent, as well as causing excruciating pain on an opponent that you may not really want to take over. It does a good job at wrecking economies.
The only backlash you may receive from a nuke is nuclear fallout, so don't nuke your neighbors lol.
Any particular reason you want to take your country out of secured mode on a peaceful world? Most people keep their main in secured mode for untouchable econ.. And build war slaves out of conquered C3s to do battle. Also, no need to leave secured mode to conquer C3s.