The standard ham radio cry of "think of the spectrum" applies here. It's great as a demo but please don't intend to use it for anything serious, this is likely outputting harmonics into areas of the RF spectrum that people care about.
I have a raspi zero whose only job is to set its clock frequency to 104.1 MHz and modulate it with Daft Punk’s Daftendirekt every morning for a clock radio that I picked out of the trash.
Well below the 5 mW power limit but it comes through clear in the same room.
Edit: for the first week I was broadcasting over the local NPR station. There were likely people on commutes nearby getting interference from me, so I switched to dead space.
A transmitter like this will radiate harmonics, and that's the major concern. It is not just a TV signal.
All that said, this is nice work. Analog signals are a lot of fun.
NTSC going away makes me sad. I have abused it to great results many times over the years.
Even a ~1W radio can go a long way, even in a city. So, yes, think of the spectrum.
Regardless of the power level, thinking about spectrum and trying not to create harmful interference is important IMHO. I live in a place where listening FM in stereo mode is impossible due to interference and radio transmissions around me. Even Wi-Fi cannot penetrate beyond next room.
We've looked how spectrum looked here with a listen-only SDR. It wasn't pretty. My friends' exact words were "Dude, that's not what I see at home. This is some serious traffic."
This is a bit bashed GPIO pin on a 3.3V microcontroller that has a max current rating of 12mA per pin. Even if that piece of wire used as an antenna here were capable of loading the pin up to its max output, it'd be something around 0.004W - and in reality it'll likely be an order of magnitude or two lower than that. That's at least 8 orders of magnitude less power, and even allowing for the inverse square radiation power pattern, this this is only gonna be detectable at single digit meters in all likelihood.
This _is_ a bad idea, spectrum noise and interference wise, but it's unlikely to have enough power at either its intended transmit frequency any of its nasty harmonics to make it outside the rooms it's in. You probably might not want to take this to hospital with you, or on that camping trip near the radio telescope, but I wouldn't think twice about firing this up in my home workshop...