I am using A-138n with my medium sized modular setup. So far I did not use any mixer which felt very limiting to me.
Pros first. It is very neat little module. With only 4hp it takes very little space which is absolutely nice. Knobs and patchpoints feel very solid. It also has tons of gain. One can find a lot of creative potential with not one, but two mix outputs.
What is most important to me - it works nicely with both audio and cv signals. That makes A-138n super versatile option especially for people with small or medium-sized setup.
On the cons side - it has too little space between the knobs. Or maybe I have too big fingers :). Anyway, it is quite rational if you need a module with such a compact size.
I am very satisfied with A-138n. It is an exellent price-quality ratio product. Highly recommended!