There are many centralized web services that operate around the world with high security. There's a large array of conventional techniques for securing servers. Admittedly most people and companies are bad at this.
I know. I have run one for over 20 years. My solution --- build a unique, custom, single purpose web server software that only supports a very strict set of functionality after every single request has been thoroughly vetted.
I see the attacks mounted against my server on a daily basis --- mostly focused around canned, off the shelf server software and the endless flaws and shortcomings in either the software itself or how it is configured.
A server dealing with money will likely attract increased attention and sophistication.
Admittedly most people and companies are bad at this.
My point exactly.
Security: The centralized server is an open invitation to hackers.
Cash out: Man does not live by web cash alone. The often overlooked issue with crypto in general which adversely affects it's utility is how to convert back to fiat and at what cost. If this can't be done in a simple, seamless, low cost manner, I'm out.