Pixel art is not necessarily about placing each pixel by hand.
It's having control over every pixel, or clusters of pixels and placing them on purpose.
When you use a soft brush it automatically creates anti-aliased lines, it creates new colors.
For example, if I point at a single color or pixel in your image and ask why did you use that color there? Would you be able to answer?
If you're actually placing all the pixels by hand, like the shading on the robot, you're spending a ton of time on something that will look smudgy and unclear.
But the main factor here is that you're spending lots of time, time that can be spent on other things if you learn to shade with the minimal amount of colors.
It also looks much more crisp and it's easy to change and tweak colors and entire palettes.
It's ok to use any kind of shading method you like, but if you use soft brushes it won't be pixel art and this is a forum mainly about pixel art.
edit: I think you should spend more time on the actual shapes, this is easier to do if you use simple shading and block out areas using 2-3 colors just to get a feel for the depth.
Image examples from this tutorial, just make sure the line art is good and then start some simple shading. Dirt, scratches, details and such can be added later.
Learning to work with depth is important, otherwise everything will look flat.
The screenshot from Owlboy is a great example on depth.