COTS is great if it does exactly what you want it to do. For the sake of argument, lets suggest we want to buy a dune buggy that comes with with a 8.9mm machine gun and a specific engine and a specific comms system.
If we want it to work in isolation then thats great - we can buy it. But if we want to add a UK weapon (strip out the machine gun as we don't have said calibre, and put the GPMG on it), and we want it to have a secure radio that we use already, rather than buy lots of kit with different radios, and we want the engine to run on the same fuel type as we already use, and by the way we need it to be airportable and we'd also like to use it to go on amphibs etc.
The moment you start looking about you realise COTS reflects the opening point of negotiations, not the finished product. What it is then about is modifying and integrating it to be something that actually useful to your military, and not a botch job collection of random kit that doesnt actually work together. That would cost far more and acheive far less.