I do understand it.
The most obvious objection to the LTV is that one can spend gigantic amounts of labor making a good that has no value to anyone, such as tying knots on a cord or digging holes in a field. The person may do it with the utmost efficiency, but it still creates no value (the same can be said about a factory making a good that nobody wants to buy). Obviously, simply adding labor to a good does not create any value, as those examples demonstrate. So the "simple" LTV does not hold even at the most superficial analysis, which is why Marx came up with the very ill-defined concept of "socially-necessary" labor time, which conceals subjective judgement.
IF we accept the "social-necessary" labor time qualifier as an answer to the criticism that labor by itself does not necessarily create any value, then as I said his LTV becomes simply a roundabout and imprecise description of supply and demand.
If we don't accept it as an answer, that is, if we assume that he did not mean to use that qualifier as an answer to that obvious criticism of the LTV, then as far as I know he had no other answer for it and his whole theory falls to the ground even quicker.
I am open minded; the simple fact that I've taken the time to read this ancient and very much obsolete philosopher attests as much. And I do encourage people to read him as a philosopher (preferably from secondary sources as much of his writing leaves a lot to be desired as far as clarity and objectivity go). But to try to apply his economic thinking , which was already quite flawed and old in his day, to our 21st Century economy is just... ridiculous. Which is why nobody does it, outside of very restricted and obscure circles in the academia and internet (this is the truth, like it or not). It's cult-like.