Now the honest part. A custom system loses on several fronts, and you should know them before you decide.
External collaboration
If your clients already live in monday.com, and you share live boards with them as part of your service, that's real value you'd be throwing away. Building a client portal is possible, but your clients then have to learn your portal. Some will. Some will complain. Weigh that seriously.
Instant gratification
A custom system does not exist on the day you decide you want one. You'll be interviewed about your process, you'll review screens, and you'll test things that aren't finished yet. monday.com is usable this afternoon. That difference matters most when a specific project is on fire right now.
The marketplace
monday.com has hundreds of prebuilt apps and native connectors. Custom software connects to anything with an API, which is a way of saying software that allows other software to talk to it. But each connection gets built deliberately. There's no app store where you click install.
Mobile polish
The monday.com mobile app is well made. Push notifications, offline behavior, and camera uploads all work smoothly. A custom system can be mobile friendly in a browser, and it can have a real app, but matching the polish of a product with a large dedicated mobile team takes deliberate investment.
Somebody else's roadmap
When monday.com ships a new feature, you get it free. Nobody at your company decides, scopes, or funds it. With a custom system, every improvement is a request you make and a change you own. That's control, and control is work.
Migration is a project, not a copy and paste. Board exports come out as CSV files, and file attachments need a scripted pull through the API. Years of item updates and comment threads often don't survive the move in usable form. Plan for what history you truly need and let the rest live in an archive.