Looking at the installation instructions, the Rhea claims to require some soldering. The Phoebe doesn't, but still requires disassembling the system. They also each only work on specific versions of the hardware (20- vs 21-pin), and which version a specific Saturn is may not be obvious without disassembly.
The nice thing about this new solution, even ignoring that it furthers public understanding of the hardware, is that it's a simple module that plugs into a slot already available and accessible on every Saturn ever sold by SEGA (presumably it won't work on the Hi-Saturn units made by Hitachi, as they had the MPEG hardware integrated, though they are also very rare and very expensive).
The Rhea doesn't actually require soldering as Dominik will do the soldering first before shipping. For sure having to know 20 versus 21 pin is a pain. But the Rhea/Phoebe have a huge advantage of being here now and known to be very reliable (they work flawlessly). Not discounting this new approach, it seems very promising, but just pointing out there's already a solution available today for those who didn't realize.