How might retaining too much heat be a disadvantage to an organism?

Too much heat can disrupt subcellular structures such as membrane proteins, and enzymes while damaging the integrity of the plasma membrane.

Explanation:

Proteins are macromolecules made up of amino acids. These fold and bond to form several biologically important molecules. For instance, enzymes, a special type of protein functions by providing alternative reaction pathways. They work best at a specific pH and optimal temperature range, beyond which they become non-functional  or denatured entirely.

The denaturing of proteins occur at high temperatures, and alter protein folding, bonding and other types of behavior. Furthermore, membrane proteins are  important for regulating cell function. If these are disrupted, the system cannot maintain homeostasis i.e a constant internal environment.