If you keep the same deflationary money over the years, you are making a profit. So what's "good" is not a money which supply grows or shrinks, but a money with which people can make good predictions and plans accordingly.
Plus, burying your money is withdrawing it from the money supply, maybe it has effects like "giving it to everybody" ? (added the risk that you can dig the money and put it back into circulation)
The argument against [sustained] deflation is that investment will be reduced along with consumption. In order for investment to take place, it is necessary [though not sufficient] for the investor to expect to earn a money profit as a result. In the case of sustained deflation resulting from an economy being tied to a fixed supply of money, the expected money return on an average investment over a time period is negative (especially after adjusting for risk) and the expected real return to burying money in the garden is positive. Thus the money stock is more likely to be buried in the garden and less likely to be invested in production, with the result that less stuff is produced and sold than otherwise could have been.
The best thing that can be said about Hayek's argument as a defence of this is that he points out that profitable companies confident of still having a market in future might still be inclined to invest money in cutting production costs (which is true, and necessary for the deflation to be sustained, otherwise you can expect prices of goods to start rising again in future as a result of greatly reduced production) but there's no reason to expect that level of investment to not be lower than an economy where nobody is incentivised to hoard.
(the original paper is included in this compendium for those interested http://www.hayek.sk/wp-content/uploads/2012/12/hayekcollecti...)