Soft solder will work just fine if the joint to be soldered is prepared right.
Soft solder does not make a good void or crack filler. In that role it is extremely weak.
Solder will cold form, swage and move around if sufficient pressure is placed upon the part(s) that it is holding and the solder in more than a tinned coating. That's where the joint can get loose and fail.
The sweat solder joining of ribs or other mounts must be a close metal to metal fit for the process to work to it's greatest strength.

I've used 50/50 lead-tin for the last 40+ yrs to re-rib mostly SxS shotguns and attach so called 'Solid ribs'. Also used it to repair and re-rib more than a few DR's over that time.
If done right, it works fine.
Do a poor job of fitting and/or a poor solder job on top of that and it will fail.

Epoxy has been used for yrs to attach ribs with. PolyChoke started using it on shotguns back in the 60's and even the stuff that was available then kept the aluminum ribs they sold in place even on the hard shuffling shotguns like the A5.

Marlin, when they re-introduced the LCSmith in the late 60's early70's attached the ribs to those already polished and hot blued bbls with cheap epoxy. The ribs are aluminum on those and they were sourced from PolyChoke IIRC.
The same brown quick mix epoxy that was used to attach the ribs was then used to 'glas-bed' the butt stock and forend of the guns.
Quality showing from every angle. But they held up if the parts were clean and the epoxy was mixed right.
How they are doing today I have no idea, you just don't see these editions around anymore to tske look at them.

They had bigger troubles with the bbl's at the time of mfg'r anyway and more than a few came back in and were involved in lawsuits with burst bbls at the chamber(s).
An entirely different matter from the above of course.