Soldering is not a good solution for long term. I was thinking the same and talked to a few guys with experience.
The final resault was that all of them said dont solder directly to riser ord PSU. There are always micro vibrations and soldering will broke in longterm.

For Riser I think molex are much better than SATA or other. I go directly from MOLEX of my PSU to MOLEX on the riser never had issues after 6 month.