you are wrong. unity free does support custom shaders.
and i find some of your points a bit vague.
If we go a litle deeper just take a look at Unity API and A7 API and the way to use it, lets say in vector calculation..Its much easier to work with A7 and Lite C that with Unity and C#.
why? i find the vec_* commands a lot clumsier than being able to use operators for example. with operators you get shorter and more readable code for vector calculations.
Now try to explore unity in vector operations....code a little and put some pressure in the engine...do some experiments and compare the speed of execution...you may find something interesting.
what may i find?
Yes..you can create your own stuff in C#, and improve the speed...
hm... why would doing it yourself in C# be faster than native methods?
