there's a bunch of common answers to this question. as it is those answers are generally:
However even though the popular answers to the question are such, that doesn't mean that there isn't actually a good reason not to buy and hold LETFs. there are a few possibilities here, and it's not all mutually exclusive.
if i had to intuit a reason that was being deliberately withheld, what comes to mind is that they generate systemic risk in proportion to the percentage of total holdings they represent. and that such risk is not borne by the individual holders but by everyone in the market irrespective of whether they hold the LETFs. if this is so, then it would be an entire/y unconvincing argument to any individual not to hold LETFs. Tragedy of the commons.