I'll tell you the pros/cons of mouse only and why people use mouse/kb and tablets.
Mouse only Cons:
1) Expect massive mouse drift (aka. mouse positional displacement from your original position, which often leaves your arms and wrist in an awkward position).
To help with this, you're going to need a very light mouse and/or a very large comfortable surface, and a lot of control/muscle. Also, if more than anything, you're going to have to learn to play with your hand in awkward positions.
2) Speed at which you can click.
It's tough to aim with a mouse as it is. It's tough to aim while clicking, which causes the massive mouse drifts. It's worse when you try to spam your clicks and ALTERNATE clicking while aiming. Ideally, you want a mouse with very light weight switch and a good comfortable grip that will allow you click very easily.
3) Combine the two above factors together, and you're going to have a bad time on streams, small cs, and SPACED Streams.
Mouse only Pros:
1) Everything is on one hand.
Less mental processing and you don't have to divide your attention to your keyboard hand, but in the end does cause more strain to your mouse hand.
2) Mouse switches are generally more responsive than keyboard switches (unless mech keyboard). Also faster to click than a keyboard.
So why do people not main mouse only and go keyboard?
Mouse drift, streams/high bpm, hand strain