I’ve built or been involved in building 4 closed source media asset management systems ( including my current place of work and one I cofounded) and you can get relatively far in the first month.

But the devil is in the details and most people that attempt it don’t even get the database models right on the first or second time, they underestimate the complexity particularly when it comes to video and struggle when the use cases widen. Document asset management systems are way easier but usually don’t understand media as in depth as a media asset management system.