I cannot argue regarding the drift or repeatability of the FX-120 when I have not tested my scale a FX-300 (same thing) against a
more accurate scale.
I believe that 
@PracticalTactical is correct on the FX-120 after all it is a $600 scale and there are scales that cost
many thousands more.
Now in a practical sense here is some recent 223 data that I have.
I picked 223 because of all the calibers I load for it was very challenging trying to get low ES numbers.
Rifle is a Rem 700 with a Criterion remage barrel , 75gr Berger, tested on three different days .
Question is, would my ES improve with a better scale?
9/8/2021
Stats - Average    2836.65    fps
Stats - Highest    2838.55    fps
Stats - Lowest    2834.14    fps
Stats - Ext. Spread    4.41    fps
Stats - Std. Dev    1.75    fps
      
Shot ID    V0    V10
1    2839    2818
2    2837    2817
3    2836    2815
4    2834    2814
5    2838    2817
9/17/2021
Stats - Average    2828.58    fps
Stats - Highest    2834.48    fps
Stats - Lowest    2821.29    fps
Stats - Ext. Spread    13.19    fps
Stats - Std. Dev    4.52    fps
      
Shot ID    V0    V10
1    2832    2810
2    2829    2808
3    2834    2813
4    2830    2809
5    2824    2803
6    2821    2801
7    2828    2807
8    2834    2813
9    2825    2804
10/29/2021
Stats - Average    2814.95    fps
Stats - Highest    2823.79    fps
Stats - Lowest    2811.42    fps
Stats - Ext. Spread    12.37    fps
Stats - Std. Dev    4.61    fps
      
Shot ID    V0    V10
1    2812    2792
2    2813    2789
3    2824    2798
4    2814    2793
5    2812    2791
6    2822    2790
7    2818    2796
8    2811    2790
9    2811    2789
10    2812    2791