I'm no structural engineer by any means. I get all my info either from Mark at Express Composites or from several seasoned boat builders, who combined have 37 years in the business (trustworthy). And, YouTube (taken with a grain of salt: trust but verify!)
That said, I understand that the 'core', weather it be wood or hollow plastic, is just a means to hold the resin and glass in place until it's cured. Whatever the core is, if it is removed the soundness and stability are still present in the laid-up glass, as witnessed when a boat is still functional even if the stringers and transom are disintegrated by rot. Using Plascore on a transom was an option in my project, however it would have taken a much more elaborate design to accomplish the end result. Plascore filled with resin does not have as decent a crushproof characteristic as wood does where the motor bolts pass through and clamp the motor to the transom. Resin shatters under stress. I would have had to ream out the area and fill the area with a solid product like composite, wood or aluminum to allow the motor bolts to be tightened correctly. I don't have the motor yet, so I don't have the exact placement of the bolt pattern.
Solid core is an option, and there might be many composite options. However, it is very expensive compared to wood; my transom would have run well over $300 for the materials.