Main advantage is you can do everything yourself the way you want it.
Main disadvantage is you have to do everything yourself.
It costs a ton to build software. Even if you’re doing it yourself, you’re doing it at the expense of other things you could be doing to more directly help your business. The company I last worked for employed four developers for months to custom-build a CMS and e-commerce application, which I inherited. They used it for some 10 years before I encouraged them to migrate to a new architecture.
They moved to Magento, so I ended up switching jobs.
If you can use off-the-shelf software, it generally behooves you to do it.
There are lots of domains that don’t have solid, mature software applications readily available. In this case, you have no choice but to build.
Do a ton of research before sinking your money into custom software. Many shirts are lost to it. Research is always going to be cheaper than development.