We need a little more detail please.
Is there a crack on the front or rear subframe?

Conversely are you asking about a floor repair, above the rear subframe bushing mounts?
Unfortunately known as a rear subframe repair, when it actually a rear floor tear.

 

Unfortunately known as a rear subframe repair, when it actually a rear floor tear.

yeah the famous e46 subframe tearing problem is poorly named. because it's really a chassis weld problem. i like how redish calls it a rear access carrier panel (RACP) repair instead. he sells one of the best repair kits, but i've also seen kits from turner and bimmerworld. some people say you can epoxy glue the plates instead of weld. also there are a few preventative measures such as the mason x-brace, powerflex subframe bushing inserts.

http://www.redish-motorsport.com/RedishMotorsportE46RearSubframeReinforcementPlateKit.html

https://www.bimmerworld.com/E46-Rear-Subframe-Floor-Reinforcement-Install/
https://www.bimmerworld.com/Suspens...g/Reinforcements/BimmerWorld-E46-Rear-Subframe-Chassis-Reinforcement-Kit_2.html

https://www.turnermotorsport.com/p-91-e46-rear-chassissubframe-reinforcement-kit/

https://www.powerflexusa.com/shop/p...shop/product/pfr5-4614-bmw-e46-3-series-z4-x3-rear-subframe-bushing-insert-3626
https://www.powerflexusa.com/shop/p...pfr5-4613-bmw-e46-3-series-e46-m3-x3-z4-rear-subframe-rear-bushing-inserts-3747

https://www.masonengineering.net/products/bmw.php?bmw_id=e46

 

The actual subframe does have weak points and does break/crack if stressed. Weak points being the rear antiroll bar mounts, rear differential bushing mount. These are fixable by welding reinforcements. Garagistic makes a nice set of antiroll bar mount reinforcements, where the rear differential bushing mount can be reinforced by adding a strip of metal around it and welding it.

Trailing arm pockets also seem to break and garagistic sells reinforcement for them as well.

The redish kit is the only one I've seen that covers the most area under the rear floor and has holes where you drill the chassis' first metal layer to reach the actual mount, and then weld there. It can be combined with the epoxy-based upper RACP repair kit (aka VinceBar) for super strong reinforcement. All of the above documented here:
https://forum.e46fanatics.com/showthread.php?t=1255643

 

How hard?

If you are good with dropping the entire rear sub-frame, driveshaft, and fuel tank, are good with cutting out the rear floor of the body work and welding a new one in, I guess it's not that hard. If you're not, well, hard is relative, but it's pretty involved. I've been wrenching cars for a long time, built race cars, etc. I wouldn't want to do this myself on a daily driver.

The weld-on plates are good to reinforce the mounting points IF you don't already have a tear. If it's torn, do it right IMHO.