Can you provide any source for the "can't use it under 30Mph" and "You can fry them" statements? The GV is practically indestructible and lugging it at 30mph in OD should not damage the GV unit. OD and slow speeds would more likely cause your engine to stall if you did not make enough horsepower to keep the vehicle moving.
As stated, in my original post, I'm not sure where I saw that but it was recent. Most likely (EDIT: it was GYC) it was on GYC since I've been flipping through episodes lately. MW claimed they went through a couple of them and the info came from GV's. I "believe" it has something to do with the lower pressures being generated by the pump at low speed?
To be fare,
1. It's the only time I've heard of this.
2. I have never researched it, so it might be stated elsewhere.
3. I don't have any interest in one due to cost/ROI for MY needs. i.e. no dog in the fight.
4. Lots of people use them without incident. Sweet...
5. It's hearsay...
Now, that all said, as stated, I have no practical experience. I mentioned it so the OP could do some research and decide for himself. If it's a real thing, then it's a real concern and something that needs to be understood to ensure the longevity of the unit.
For "Me" it seems like overkill when other options are more readily available unlike in the past. They fill a need/gap for some people which is great. That's what a product like this is supposed to do and as far as I know, they do it well.