Interestingly enough, this particular question has been tested by Rtings [1] They found that during their tests, inverters which continuously varied the power did not necessarily lead to more even heating, but they do note that small quantities of food (like the butter you mentioned) that need to be heated for very short periods show some differences between inverter and non-inverter models.
Overall, they found that the improvement was much smaller than I'd originally anticipated.
Overall, they found that the improvement was much smaller than I'd originally anticipated.
[1] https://www.rtings.com/microwave/learn/research/microwave-in...