I have fed my Growatts' ac inputs from all kinds of ugliness including an MSW 120v inverter fed through an autotransformer to make 240v. It even sounded bad. ? I've also stalled a 5000w generator with the Growatt repeatedly, and tripped a 17a(@240v) thermal breaker on the generator repeatedly while experimenting. I've also manually slowed the generator down to ~50hz just fine. They have accepted and used everything that fell within the voltage/frequency range stated in the manual. I haven't had any trouble getting them to charge from the ac input, which makes me wonder if this reputation you mentioned is based on people just having trouble with the settings.
The only thing I haven't gotten them to do is start grid charging from 120v. Which it doesn't say it will, granted! But it does say it can accept from 90-280vac and even gives a graph of how it derates from those lower voltages. And i have fed it 240v, let it start charging, and then rapidly switched it to 120v and had it continue charging from 120v. So i wish it would just let you charge from 120. But since it has to go into AC bypass to charge, i understand why it doesn't 'allow' that.