So, for one, a convective sigmet is only useful for telling you the weather might suck. No one except maybe the military gives any fucks about flying in an area covered by a convective sigmet. What you do care about is how angry the weather is enroute, at your destination, and the capabilities of your aircraft.

Not all storms are created equal, and neither are aircraft. There was plenty of stuff I'd happily fly a BE99 in that I wouldn't touch in a Cessna 210. There's plenty of stuff that looks incredibly angry on the radar that turns out to be nothing (southeast thunderstorms versus great plains thunderstorms, as an example). Unfortunately there's not really any way to know that without just diving in and seeing it up close.

This is not to say you should ignore regs. If your plane isn't rated for icing, you don't go into icing. If you don't have the equipment to shoot a 500rvr approach, you don't do it. No company can fire you for refusing to violate FARs. However, they will absolutely find a way to can you if you are constantly cancelling flights that are perfectly within reason but you're afraid of shooting an approach to 1800rvr or there's a little snow enroute.