Expensive? Noooo, it's not! There are plenty of us today with integrated adc 12 bits or higher. Take one that is fast enough. The cost of the Micro is already included
If we calculate with a conservative (fluke 87) 20 Khz bandwidth for the TrueRMS core, you need to take samples at around 200KHz, otherwise it is not really true rms. An ADC with those specs there is the ADS8339 which is 3.5 USD from TI. You need to buffer and amplify the signal. I will assume the AD8436 is getting a 1V signal, so the amplification is only 5. Assume 0.5% max error, you need an opamp doing less than 1mV offset, 2MHz GBW. Something like a OPA197 can do it, 60 cents. You will need to dedicate an SPI port of your micro to it. Also coding needs to be done, verifying that the RMS calculations are correct. Luckily, there is only one "root calculation" in the code. I think with the usual big company impotence, a few months should be enough to develop this.
Compare this with the price of the AD8436, which is ~3USD. You gain nothing, but more chance of error for more money, and more code. While you could just solve it with an IC. #analogisbetter
At this particular chip, I would suspect the following:
At low power supply voltage, over the temperature range, it might have linearity errors. Maybe they had problems with the output buffer going only to supply voltage -1V. Sometimes they just dont test the part with a certain scenario, because they haven't thought about that.