Well, being a developer you'll know exactly the amount of time, and effort that has to go into a product to make sure it functions as expected, and doesn't throw any sort of problems at you, and even then when you come to testing it there will be errors, and all kinds of different issues which then have to be redone, and sorted out. The thing is, most of these scripts are amateurishly done, are churned out as quick as possible, and sold to the maximum amount of people as possible often without any kind of support. This should be a red flag by itself. Their development team could probably be counted on two hands, and they claim they offer lifetime support to the tens of thousands of people they've sold this exact script too.
I wouldn't trust or use a casino that was buying scripts that have been sold by the dozen. The only scripts that come to mind is from random developers that usually don't have any real world identity, and therefore lack the support, while still claiming that there's a lifetime guarantee. There's also these white label companies, which from my experience have been nothing short of horrible to deal with. Usually, just as bad support, bad documentation, and whenever you need to make a change, which you will need to make a change if you even remotely want to have your own proper brand, and custom games you'll be charged from the nose to get it done.
I was always taught that when you're coding you aren't just providing that code, but you should be offering support for that code too. That to me is one of the biggest drawbacks to anything, but tailor made software.
Plus, you want to be absolutely sure if your running a casino that you are handling your customers money with the absolute highest standard possible, scripts are unlikely to provide that. I'm not just talking about depositing, and withdrawing, but the issue with any bugs that could effect customers. There's also the issue with scaling, I can guarantee most scripts haven't considered scaling it if in needs to be done in the future.
You basically pay huge amounts extra for customization and you are restricted to what they offer and not necessarily what you want. If the changes that you requested was previously requested by another casino, then those changes are done quickly.. but for "new" requests.. you must be prepared to wait for months.
Exact same experience with other unrelated programs. Like I said, nowadays I look for companies that can provide not only the service I want, coded to my specifications, but the required manpower to actually offer any support whenever it's needed. Nothing worse, than looking at some code which has ditched all the principles of coding, and is just an absolute mess. Even the most seasoned programmers will find it hard to wrap their head around it if their code is messy, which probably tells you that they also will have a problem deciphering their mess.