We can observe an object with mass as it has a well defined boundary but it is not the same with energy.
What may happen to the information or data that exists with an object and is needed to constitute an object when mass is converted into energy.
When we see matter as objects around us it has details. For example a ball has structure of elements and molecules constituting it, its shape, size etc. If suppose it was converted to energy it should still posses the information about its constitution, its identity as information cannot be destroyed.
And when we talk about other objects and forms of matter that are complex, say those with life. They have much more information associated with them. What can possibly happen to all this information if suppose it was converted to energy. There has to be some way to store all the information associated with an object be it any form. A energy counterpart for every particle counterpart? For example if it were a chemical reaction and water vapor was converted into water the molecules are still intact.
When there is a nuclear reaction and mass is converted into energy the information associated should still be present in some way.