I get that what is good about it is that you have a very low trigger current, which is especially useful if you wanted to trigger it with say a 555 timer or something, or where you have a big boy plane with a panel drawing 25amps and a 170 amp alternator bolted to that Lycoming 0360. But I am talking about ultralights here. The total output of the alternator is 17 amps. That's the total, but all the loads add up to maybe 5 amps through my (described rather than named because I admit I'm guilty of confusing terms) OFF/ON/START switch. But if for some reason I thought that was too heavy a load for that switch (which it isn't), why wouldn't I just use a 35amp automotive relay? That will have a switching current of maybe 200 mA. What is the SSR? 50mA? But it's constantly dumping power. So yes, only 50mA through the switch, but several amps as heat through the heatsink, while the 35amp relay is only losing 200mA total.
The original query was about total isolation of the battery. To do that needs something that can handle the cranking current of the starter. Which is either a giant relay (which is why I started the thread since I hated the idea) or a giant switch AKA a battery isolation switch.
Gotta keep it simple on an ultralight, especially with only 17 amps to play with 🙂