The problems that exist for chiropractic are that:

1. Subluxation theory is only theory.  Attempts to prove it as fact have been deemed unsubstantial.  There simply is no valid high-quality scientific evidence that supports that a subluxed vertebrae can cause disease.
2. The profession is divided at least into three associations, each with its own beliefs about their role.
3. It lacks acceptance in mainstream medical models.  That is not every traditional physician or insurance company recognizes chiropractic as a primary health practice.
4. The highest quality research trials conclude that x-rays are near useless for back pain (source: The BackLetter).

Chiropractic, however, can work.  Why?  We're still not sure the mechanism of what really happens with manipulation.  Several theories exist, but none that have stood up to scientific proof to date.  All that we know is that the phenomenon of manipulating a joint and a resultant improvement of a symptom can occur. 

For PT, somatic dysfunction is evaluated based on cluster signs & symptoms, performance of critical tests, a good patient interview, and the integration of this data with the way that the patient moves functionally. 

A PT would want to know how driving, sitting, getting up from a chair, walking, twisting, etc. all effect a patient's symptoms.  From there, a PT would discern what type of movement impairment was occurring and devise a strategic treatment plan comprehensive to reverting the patient symptoms and recovering functional abilities. 

To keep it simple:

Chiropractic thinks Structure.  PT thinks Function. 

Structural problems based on subluxation theory causing disease is purely arbitrary (opinion or belief system-based). 

Function is undeniable.  If you can't sit without pain, you can't sit without pain.

Period.