@luckystrikes 

It could also be that you are just used to using traditional routers, so you find the Web UI easier to use because it is more familiar.  Can the app be improved?  Sure, but you can say that about anything, really.  There is always room for improvement.  For the Deco app, it could benefit from a power user mode.   Deco's app was designed around simplicity and the general user for its UI and experience, so some power users have commented that it feels bland or not intuitive enough. 

As for security, it's just as secure, maybe more so than a web UI of a traditional router.  For a traditional router, anyone connected to its network and who has access to the admin username and password has the potential to access the router's interface.  For Deco the only people who have access are the owner and managers they assign. each requiring their own TP-Link ID and unique password.  Plus it has the added benefit of push notifications and remote access via its link to the TP-Link Cloud.

Finally there is a GPL code available for both hardware versions currently available of the Deco BE63 so a person who has the knowledge and ability could create a open source FW if they wanted to.  Just remember while its possible using a 3rd party FW does void the warranty of the product.