Lifting low areas does not repair a poorly performing slab foundation
There is more than one way to make a slab foundation more level. For a newly placed slab, the preferred method is to fill in low areas with cementitious material and to grind down high areas. This method is almost always never used on an existing slab.
Once the floor coverings are applied, the normal method of leveling the slab surface is to underpin low areas leaving the high areas alone. This usually means underpinning using piles or piers and lifting the perimeter only while leaving the interior areas alone.
There are three problems with underpinning using piers or piles. First, it fails to address the actual cause of foundation distortion. Second, it is rarely cost-effective. Third, a stable foundation that is not level is much preferred over a foundation that is level but not stable.
Most slab foundations can be made to perform better without any underpinning
Most homes with slab-on-ground foundations can be made more stable with underpinning. The key is foundation drainage, foundation watering, and vegetation management, especially with regard to trees and large shrubs.
Some slab foundations cannot be made to perform better
Sadly, some foundation cannot be made to perform better. If you want to spend a lot of money, a thorough petrographic analysis can be made of core samples of the concrete. This may or may not yield an answer to what the problem is. In my experience, petrographic analysis is rarely used on residential properties.
Foundation drainage, foundation watering, and vegetation management are keys to better slab foundation performance
To repeat from above: foundation repair contractors do not repair foundations. They try to make your foundation more level. If correcting foundation drainage issues, a consistent watering program, and managing vegetation issues does not get you where you want to be, there is an excellent chance that foundation repair will not get you there either.
If you find that hard to swallow, read the warranties and agreements from each repair contractor. They make no promises regarding how level the repaired foundation will be after the work is done. No promise that drywall cracking will cease, that existing brick veneer cracks will close and that there will be no new cracks in the brick veneer.
What they call foundation repair is akin to a roll of dice in a casino. You might occasionally win, but the house always wins over time.