To me, the worst potential problems of the various piston AR designs are parts availability and quick repairs, especially over the long run. If your DI AR needs a part you have your choice of hundreds of vendors selling standard parts, and you can install them yourself or have a local gunsmith do it. If your Ruger piston AR (for example) breaks, you must get parts from Ruger, and they may not provide them to you - you likely will be told to send the rifle to them or an authorized service center, not you or your local gunsmith. And if a proprietary piston design is ever orphaned, well you'd better start scrounging for parts now...
And with so many piston designs out there it's fair to assume a shakeout of some kind where some designs will be abandoned. You could be stuck with Beta vs. VHS or HD-DVD vs. Blu-ray.