You have to trust the vendor's interest in making a profit. Nothing wrong with that :-)

Closed source (usually) gives you Ts&Cs, support model and some level of guarantee of fitness for purpose which is why we pay to use it. This is not to say that there aren't bugs that seem to take forever to fix, but usually that is not the case at least not when there is a risk of the product becoming unsalable.

Of course there are closed source software or appliances that just are not worth their cost because they fail to mitigate the risks of having us trust what we cannot see, but these usually fail to gain any significant market share.