I never did get Ralph Bacons board installation instructions to work, but used this in the preferences "additional Boards Manager URLs:" and it has worked with every "clone" I've tried.
https://raw.githubusercontent.com/dbuez ... index.json
In general, I've found they work fairly well and where speed is needed, they are far better, with three exceptions:- Where accurate timing is needed, some libraries won't work without modification and the EEPROM (or lack of it).
I'm not certain that the internal oscillator is actually 1%, though serial has always worked OK, so must be within 2%. When I've tried a ppm meter, some read identical to Nano with an xtal, some over 1% faster and some a bit slower, so 1500uS is timed as 1522uS and another as 1490uS whereas a Nano with an xtal will always read 1500uS +/-1uS. But it seems fairly well temperature compensated and doesn't seem to vary much with temperature.
Some badly written Arduino libraries won't run without a bit of tweaking. Some check if 8MHz and if not assume 16MHz, so are easily fixed, other libraries cause obtuse error messages that I can't trace and usually find it quicker to find a different library that works OK.
The other minor issue is the EEPROM is emulated in program flash, a bit like you can do with the Opti bootloader. If you want 1K of EEPROM, it reserves 2K of flash and splits this into two banks of 1K. If you do an EEPROM write, it copies the "active" 1K bank with the saved values to the other 1K bank and then makes that page active. Next EEPROM save and it swaps 1K banks again. EEPROM writes are normally relatively slow, but this is way slower. The bootloader is 1K, so you get 31K useable and if you use 1K "emulated EEPROM" then only 29K is left, which for most isn't a problem.
I didn't try programming with an ICSP as the Opti bootloader is fairly fast and I've lived with that.
It'll be interesting to hear what you think of them.