I would use GOG a whole lot more if the offered the ability to download untouched Isos and Disk images from games you purchased, some one els here mentioned compatibility but there is also the problem with GOG using 3d party fixes from authors without there permission or giving them credit.
Having a good Iso image would also mean in most cases that you do not need to rely on cracks to get the game to work (as most cd based copy protection can be emulated) and this will open the doors to game updates that might not have a good crack available for it.
It is however unlikely that this will ever come to pass , and as a result I am highly skeptical when buying there games, even there dos games for example have been modified and I have found some that has had there setup files completely removed from the installation , rendering them hard to use on the real system.
I think the best way to go about collecting is to rely on more conventional means like thrifting or hunting in the wild 😉
As for price, I stay in a country where most games just dont come up for sale, and when they do hit auction sites their orders of magnitude more expensive than whats available in the UK or US, and shipping isnt a option due to cost and import taxes, so having at least one place online where I can source my games legally is great, but having to struggle to make them work on a system they were intended to is not, lets face it Piracy is easier, and GOG needs to step up there game to combat it, especially in countries were piracy is the norm like here.