I'm going to attempt to draw together a few ideas.
1. The Internet
You state that the construction of this device is 'unintuitive'. However, think of how quickly we can communicate today, and how quickly things get leaked. Without serious government restriction, the plans for this thing will be easily Googleable within weeks, if not less. Even if your government does restrict this device, the plans will most likely still be available on the dark web.
2. Global Warming
Free energy could have an effect on global warming. As mentioned in the comments, all the energy created will eventually dissipate as heat, which could eventually burn Earth to a crisp. Now, say it takes 30MJ (megajoules) to start up:
$$ 30\text{ MJ} \times 300 = 9000 \text{ MJ} $$
$$ = 9\text{ GJ} $$
We get left with a 9 gigajoule output. If we take that and the heat capacity of air (which is 1.005 kJ/kg K at room temperature), we can work out the heating effect. The heat capacity tells us how much energy it takes to heat up 1 kilogram of air by 1o Kelvin.
$$ 9\text{ GJ} = 9,000,000\text{ kJ} $$
$$ 9,000,000 \div 1.005 = 8955223.8805 $$
So, we can heat up approximately 8.95 million kilograms of air by 1oK. The approximate mass of the atmosphere is $5.1480 \times 10^{18}\text{ kg}$, so if the 9GJ output is output per second, then we need $(5.1480 \times 10^{18}) \div 8,950,000 = 5.75 \times 10^{11}$ seconds or around 18,239 years to heat up the entire atmosphere by 1oK (call it 18,000 to make up for my rounding). So, if you've only got a few of these devices then the effect won't be too bad (and certainly less than global warming is now), but you can see that it could become a problem.
3. Terrorism
Sort of linked to the first point. If the plans for this are available on the internet, then it's a terrorist's dream come true. They can build it reasonably easily, it can be concealed fairly well (although admittedly it would get picked up at airports), and it does a fair amount of damage. If you want to put this device into a story I'd suggest rethinking the positive feedback idea. Perhaps you make it compulsory for these devices to have an earth or ground cable to siphon off the massive overload, or maybe you give each of them internal batteries to hold some charge. Yes, it could still be weaponised, but a weaponised version might stand out a bit.
It's a good idea, in theory. I think it needs a bit of refining, then it's ready to go in your world.