With your solution you need to manage infrastructure. Why not abstract that away?
It's on a separate network so they may go down and cause outage in your own app, go out of business, etc. (Yes, your IaaS service provider can go down as well, but that's a much larger operation and something that's very-very likely to have a much better uptime than what you could do on-site. And whatever the case is with that, it's still on top of that.)
Abstractions aren't free. (BTW, I don't see it as an abstraction, it's simply outsourcing. You'd use some kind of API anyway even if you hosted the job queue yourself.)
Depends what you're building, but likely a good trade if it's an MVP.
At $30/mo you're already ahead if it saves you 1 hour of dev time.
Most of the time, I don't want control over my infrastructure (boring, low value); I'd rather focus on building features (fun, high value).
You need to put up significant resources in order to beat a dedicated team running this. Many companies can do that. Many can't. Even Twitter increasingly runs into problems with site reliability...
It’s just scary to hand off such an important part of your system to a third party.
Also, you don't know the team size behind this service, as I mentioned in my original comment.
If these start to matter, even picking a service from a list of possible providers takes time. Learning their tools, their APIs takes time, etc. OTOH if you have a project template, a setup you know, doing an MVP with that can be really cheap/quick. If not, I can see how this makes sense. But I'm not sure you can have a sustainable business just by serving people building MVPs.