Although I don't know the details, I also think a MSU-1 variant for the MD is a strange invention when it already has CD hardware. If it really reduces the size of the patches and has more features it might be defensible though.
So I'm gonna assume this probably isn't an issue for Terraonion, and Pro as far as things go. According to them they were very careful to keep the voltages within the margins of safety. It draws less then the Virtua Racing cart.
No it's not just about power draw, it's about translation of logic levels (voltage) between parts. The SD card for example is a 3.3V part and is not tolerant to 5V which the MD is using, and the FPGA might also work on 3.3V only. So it's necessary to convert the communication between the 3.3V and 5V parts so that each part works with its expected logic level. Many pirate and bootleg carts from China (but not any official Everdrives) are skipping this part entirely and doesn't even have resistors to limit the the current, which means they are potentially dangerous to the hardware (although this varies a bit between different types of hardware). I have no idea if Terraonion's stuff are correctly built or not though.
Soon someone might post a reassuring video from BennVenn (who is in the Flashcart business for the Game Boy) where he "proves" that all pirate carts are totally safe, but I'm quite sure that that statement simply is not the truth. This is a hot and infected topic with lots of business interests and probably personal feelings involved.