This is cool, but to me it sounds like you are building high value targets for a hacker. Hack one instance and get access to hundreds of users browsing the internet.
Even in this free demo, every user has their own browser process, with its own uid owner, and that OS uid has its own limited permissions.
At the same time, it's not an insignificant risk at all and you raise a very good point, which I'm surprised no one brought up before. Thank you for bringing it to everyone's attention.
An instance is a single point of failure, it's also less attack surface. To some extent, that's a tradeoff. Relative to all devices and network infrastructure in a typical company that access the public web, there's less attack surface if all web access funnels through a BG instance. On the other hand, it's a concentration of the risks into one place. My belief is that makes it easier to manage, and that the "gap" between the client infrastructure and devices and the cloud (through which only pixels, and a wire protocol of user intent pass), makes it more secure than accessing the public web directly.
Even tho it's a single point of attack, a compromise of a cloud machine, is not the same as a compromise of a device in a company intranet, or a mobile phone of someone in the company. In order to exploit the user's local machine or their organization's network, an attacker would still need to convert any instance access they had into access of a company device or network. This could happen through attack vectors in the pixels for the screen view (less likely) or through compromising the source code that serves the thin client (more likely). This is why monitoring of source code integrity is important. Open Source is an important part of that.
At the same time, in these free demo versions the browsers only exist for 10 minutes, and, exactly as you say, hundreds of strangers are all browsing from the same machine together.
TL;DR - It's a tradeoff of centralized infrastructure. There's less attack surface, but there's also a single point of failure.
Also, if you want to responsibly dislcose any security vulnerabilities you discover, please report to cris@dosycorp.com and if you want I can acknowledge you here https://github.com/dosycorp/vulnerability-reports