If you have a web application in the back end, that's what the Front Door is for: you have to have a web application firewall.
Microsoft's Web Application Firewall is designed for the Application Gateway and regional load balancing, or global balancing with Front Door.
It works perfectly and you can assign as many web application firewall policies as you want to the same instance of Front Door.
While Front Door works on Layer 7, Traffic Manager works on Layers 3 and 4, and is mainly a DNS router. You can use either one. But, if you have web applications, Front Door can provide you with a much richer toolset, especially when it comes to security, directing traffic, and rewriting the header and the URL.
There is room for improvement and they're working on it. Every six months they come out with a preview of changes.
Front Door is a global load balancer on Layer 7 and you use it with the Azure Web Application Firewall.
Azure Front Door's stability is very nice.
The only thing you need to do is to create your Web Application Firewall files. It's that simple. You don't need to create a custom balancer because Microsoft covers that in the application.
It's a piece of cake to maintain. Usually, when I design it, I train the staff on each resource for around three to four hours. Within two to three days, they are up and running and know how to manage it.
They concentrated on Layer 7. If you go to the portal, you'll see that they put the load balancer in just one service, whether it is Application Gateway, Azure Load Balancer, or Front Door. They then ask you a question: "Do you want local, regional, or global?" and they will tell you which one to choose.
However, you could use the web application firewall with Front Door without needing it. If you are worried about the traffic going out from your infrastructure—if there is anything malicious there—the web application firewall will not do TLS. It's just for incoming traffic. That's what Azure Firewall does.