Soldering does not isolate anything from electrical discharges, it simply prevents things from coming loose.
If a person owns more than 10 machines, and has not learned to solder properly, or done some training, "there be trouble down the pinball road in the future". I am not talking about circuit board repair.
Electrical isolation requires a interrupting or bypass circuit in the form of a daughter board PCB with fuses, separate grounded signal which is built into the device itself (with no "open" wires such as alligator clips), or a combination of resistors and diodes to prevent electrical signals for power moving in the wrong direction or interference noise "feedback" in simple cases (people create their own with "green boards"). That is why some people hear "hum" in their subwoofers with alligator clips.
Hum is not a real problem, that is a sound signal unless it is REALLY LOUD.
Static, squelching, or other horrendous noises on the other hand is BAD because that is electrical discharge.
In the case of my subwoofer (which is currently disconnected, and 3.5mm end plastic shielded), a PCB daughter board (with fuses), insulated 20 gauge wire, and molex connectors.
Anyway, the summary for my case is protection at the board, protection through the wire, and protection at both ends via my methods. Nothing should be exposed. The subwoofer is already grounded.
Using alligator clips is like stripping a grounded plug and directly inserting it into a wall socket, each wire attached to its own screwdriver in terms of protection.
However, it really depends on the application on how severe a power short back to a board can be.
In the case of a static discharge or non-grounded short back to the MPU using alligator clips or other MacGyver methods, DEAD board.
I guess that seems quite serious to me.
If you are talking about a light shorting out due to a bad diode (not as much of problem with LEDs anyway), not much is going to happen unless EVERYTHING else fails upstream.
The real problem is you have to LOOK at the schematics to see where in this case Stern cut corners in terms of where electrical vulnerabilities were in the building of a pinball machine. Stern did not consider people hooking up externally powered 2000 watt sound systems when they designed their games.
The sound system IS a vulnerability.
DKPinball has made some really good 5/12v PCB cards with fuse protection for use with Stern type mods. He even did several as proper power taps directly off the main boards. I have not see anything for 50v though or subwoofers, but it has been while since I checked. Might be a good idea for him.
Finally, don't use CHEAP electrical surge protectors for external protection, they do NOTHING.
Either spend a little more money and buy something with actually overload protection like an APC, unplug your machines via master cord, or install a dedicated, electric circuit interrupter in combination with your home breaker system. You will thank me later.
Right now, I have to keep my entire "master pinball collection" unplugged because I am renting a home after my first retirement, and I cannot install another 220v master outlet with override protection.