Don't get too hung up on efficiency. What counts is signal strength.
Suppose you have a bidirectional antenna. Unless you're talking to two
people in opposite directions at the same time, it doesn't matter if the
antenna is 100% efficient and half the power goes into an unused reverse
lobe or whether it goes into a resistor which makes the antenna 50%
efficient. The result is exactly the same as far as the other station is
concerned. So to the extent that the rhombic isn't optimal, it's because
it's inherently bidirectional, not necessarily because it's inefficient.

A bidirectional antenna is usually not an optimum choice. For the same
number of elements or same amount of real estate, you can usually make a
unidirectional antenna which has a single main lobe of about the same
width but 3 dB greater gain. Or, you can have a main lobe of about the
same gain as before but greater width, which is an advantage when the
antenna can't be rotated. However, this doesn't say anything about
simplicity, which is the main attractiveness of a rhombic, along with
its bandwidth.

Roy Lewallen, W7EL