Schwartz Performance, Art Morrsion, Eckarts, Roadster Shop, et all, frames all have the same issues as any other frame car posses; as a ladder design they all twist some amount, they add weight, and are separate from the body structure so they require specialized means of attachement. Using any of these under an E body, or B body, or any uni-body car turns it into a vehicle similar to any other body on frame car, such as a Chevelle, GTO, Thunderbird, Imperial, etc. Is that good or bad, all depends on the rule set of the organization you run with and your intended application.
A full blown stiffening kit added to a uni-body can create more rigidity without the weight penalty. I also have yet to see any of these frame conversion cars out beating on them like the Hotchkis boys have done with the stock E body and select reinforcement. Can they hang with them, maybe, but I haven't seen it happen yet. However, I do see a lot of high end, ultimate style builds using frames with lots of pretty powder coating and paint work.
If it matters at all, I'd also speculate that a frame conversion Mopar will not retain value the same as a reinforced uni-body car will. I know that isn't high on some people's list, but if you are the sort that tires of a car after several years only to move on to the next build, it could be a consideration.