i actually think that the design of the vanilla icons is great. the best. trust me. everybody knows it. i work in a design oriented profession so its extra nice to see these because so much care has been put into each one. In short they are easy to see and understand and don't make your eyes bleed. comparing them to icons from other games, i really havent seen anything as good as these, both in scope and quality
WHY that happens is a little more complicated but it comes down to the relationship of the individual pixels with each other.
Since the icons are very small, here every pixel makes a huge difference, so you really do have to get them spot on, or its just bad. And gpg really did get them spot on, we can see this even now, by comparing the faf addition of the heavy bot icon. it has several "flaws":
1. its a diagonal cross out of individual pixels - those don't really work at that size, the lines are not defined well enough. it leads to it being unclear since 5/9 of its coloured pixels are forming a 3x3 checkerboard pattern in the centre. that's not easy on the eyes.
2. the central part of the icon is laid out on a 5x5 grid but really there aren't icons that take up the very corners, because of the way they start to intersect with the edge of the icon, we can see this happening in the nomads t3 heavy tank - the X goes to the very edge.
so what i did is change it a little into what you see above.
1.it has a quite simple shape with no diagonal interactions, makes it easy on the eyes
2.it doesn't go to the edge of the diamond shapes anymore
3.its a fairly static image which i think its appropriate for armoured bots
4. after making it i realize that its also in the shape of an H which is the first letter of Heavy so that's cool i guess xD