We use a truck mount for most of our upholstery cleaning, and its what I recommend in my classes if you already own a truck mount.
Fabrics with very sensitive textures, such as velvet or chenille made from silk or rayon, could be more safely cleaned with a portable machine, but skill is the most important factor with such materials. You can damage such fabrics as easily with a portable unit if you use the wrong tools or products and don't know what you're doing.
Truck mounts have the potential of causing more damage based on the amount of water that could be used, and the possibility of fabric damage from the extraction tool and unregulated vacuum. Both of those problems can be nearly eliminated with the use of the appropriate cleaning tool, training, and common sense.
As far as chemical choices are concerned, portable equipment does allow for more variety when it comes to extraction products, but most upholstery cleaners stabilize dye and prevent browning with acidic rinses and fast drying techniques, and only change the preconditioning product based on the soil level and fabric or dye sensitivity.