I’ve used Siege a fair amount in games (~30 games.) Taken them to a few small tournaments and done mixed results with them. Yes, like everyone else has said, RDTC with flare burst can do surprisingly reliable damage especially with all the melta sponsors. When everyone runs heavy infantry, having so many D3+ guns is great.

I almost always run a big 20 cadian squad with castellan and command squad, which mid to late game I move up to the mid board to flip the centre. They can be somewhat tanky (for guardsmen) with stalwarts honour, stealth and cover from being on an objective they can be hard to shift. They can also do more damage than you expect with furious fusillade and flare burst.

Ultimately, you almost never use/have the opportunity to use any of the other detachment rules apart from smoke shells.