I've seen a lot of spin bowlers on the international scene, and perhaps there hasn't really been a leg-spinner like Shadab Khan on the international scene.
A leg-spinner with a load up of an off-spinner, one whose stock leg-break does not get much spin on surfaces outside Asia or even much in Asia, and a player who prioritized more so his batting to become a more favorable asset to the team rather than settling down as a genuine wicket taking bowler to compliment a fast-bowling attack that was working its haunches off (before anyone talks about the recent form of the pacers, one has to remember that our pacers would not be under such immense pressure if the spin department could hold itself and step up from time to time).
I think that managements that we have had did not communicate the needs of the team to Shadab Khan. It kind of led to a player who was constantly in the ropes of national selection without really having to improve or change his bowling style. In T20s, he is an incredibly favorable spinner, because his wicket-taking largely occurs from players taking risks and getting out.
However, what I would certainly be interested in seeing is how often Shadab Khan strikes a bowled or LBW dismissal. The reason being that as a spinner, his deliveries rarely end up on the stumps. For a spinner who does not extract too much spin (this being mostly due to the fact that his rather linear load up forces him to strain his side and lose his head position to impart more revolutions), this then means that on Asian conditions, it's quite safe to play risky strokes because LBW and bowled are out of the equation.
There is little element of risk involved, and all the more potential for batsmen to profit off a mistake of his rather than look to take a chance off a good delivery. So from a technical standpoint, his first failure is not being able to keep the ball in the stumps.
Secondly, his bowling mindset is too one-dimensional. A bowler should be making his own field placements to try and take wickets and build pressure. Shadab Khan follows the T20 bowling mindset - he is letting singles and doubles occur due to fear of conceding a boundary. In ODIs, a spinner has 60 deliveries to conjure wickets. A bowler needs to have the patience to set up batsmen in their spell, exploit any spin available on the surface, and use natural variation along with their own variations to cast doubt in the mind of a batsman.
Take for example Kuldeep Yadav or Adil Rashid. Both are bowlers who toss the ball up in the line of the stumps, making use of angles, drift, and natural variation wherever applicable. They bowl slower than Shadab Khan, but more so, they don't rush their overs. They build dot balls and keep pressure on the batsmen. They try and set up batsmen intelligently, forcing them to take unnecessary risks whilst also having the skill to deceive batsmen in front of the stumps.
For Shadab Khan to make himself into an ODI bowler, he needs a better bowling plan, he needs to strategize and create opportunities by depriving batsmen of runs. He needs to get players on the front foot and bowl according to the field.
Usama Mir isn't really a great improvement over Shadab, though he gets more spin from the pitch, he also lacks control. But perhaps he is the best option for Pakistan at the present moment.
If Pakistan had Abrar Ahmed in the setup, I would have tossed my hat in that corner. At least a test match spinner has that pinpoint accuracy and mindset to set up a batsman. We need a patient spinner who bowls wicket to wicket. He has the mystery too, would have been a good shout.