We split these functionally into

Print Placement Reflow Carriers (PPRC)
Placement Reflow Carriers (PRC)

The main difference being that a carrier used for printing cannot have location pins
or fixings above the upper srface of the pcb.

Against these differing process requirements we have engineered designs which we think is the best compromise between

support
location
retention
thermal management
ergonomics

The viability and feasibility of using SMT Carriers for each particular board is difficult to evaluate. You basically use them when

You have a bottleneck in placement
You have odd shaped cards
You can process both sides at once (most times - it saves a fortune)

We will gladly examine any board (using Gerber) that you have.