See, the thing about a market economy is that the value of assets is difficult to ascertain but can be "discovered" through the actions of market participants. Sometimes, a whole bunch of assets are tied up in a company that seems to be using them well enough, and might even be generating a profit (on paper), but in fact those assets could be used even more profitably in some other productive capacity.
Note that this doesn't restrict assets to PPE (property, plant and equipment). I'm pretty sure anyone who has ever been an employee can attest that human capital can be just as unproductively employed as physical capital, but still show a profit on paper.
So no, you can't make some blanket statement that all companies are worth more than the sum of their parts. Some might be, but I believe the main reason we are a capitalist society is because allowing markets to freely discover the prices of assets results in a much more productive allocation of capital than the alternatives.