First, you need to write the cronjob. But what goes in there? You need to decide exactly how you're going to make a backup, and the process may differ by what's being backed up. Ideally you want a quiescent snapshot, but the way you do that varies by application. What if the application is a distributed application, in which case you need to synchronize the snapshot process among all its nodes? What if it's a master-replica design, where the node that runs the cron job may vary based on the current topology?

And if you need some sort of cluster-aware lock to coordinate backups among different peers, you'll need to decide which system works for you, implement it, and maintain that as a separate system. And if that needs to be upgraded, figure out a bulletproof process for upgrading it while it's still being used as a coordinator.

Then, you need to ensure there's storage for the backup. You need to decide what kind of storage you're going to use, make sure you've got enough space, figure out how to encrypt the storage (very important in secure environments), how to protect the storage using authn/authz. And lots of environments have retention and storage lifecycle policies - you don't want to put the old backups on the expensive fast media; you want it on the cheap slow media. And some environments make you dispose of old data, so you have to figure out how to age it out but without ever losing the backups you want to keep.

Finally, you need to make sure the backups you create are valid and usable. So you'll want to build an automated regression testing procedure to ensure that every time you make a change (regardless of how minor) to the system being backed up or the backup process, that you end up with usable backups.

(Disclaimer: I work for AWS, but opinions expressed here are my own and not necessarily those of my employer.)

There was a time when I used to think the same, then I found my backups were corrupted (or stopped because run out of space, etc) just in the moment when I needed them.

Not for a reliable solution. For example, assume you have a master and a replica database for reliability, what happens if the master, where the cron runs, fails? Do you remember to set up the cron up on the replica? From my experience, having worked on backup software, over 10% of servers that need to be backed up are not.

System reliability is hard, and the cloud makes that easier.

Need a lot of storage and make sure the backup is readable (view the files content or try a restore).

The number one backup solutions nowadays is AWS S3, because it's easy-to-use unlimited storage.

How does a company handle backups without S3? Usually they don't. That would require employees to buy machines/SAN with tens of TB of storage and maintain them (weeks in ordering and travelling to the datacenter once in a while). It's too much hassle so nevermind.

How much liability can you claim against Cloud Employee if there's an issue with your RDS backups?

Liability? Probably none. See section "11. Limitations of Liability." here: https://aws.amazon.com/agreement/

RDS SLA's are here (doesn't mention backups though, so not sure how that's handled): https://aws.amazon.com/rds/sla/

Not a lawyer or anything, but my layman's understanding is that, you essentially are voluntarily opting-in to waiving liability when you sign up for AWS and accept the terms and conditions, and instead of liability, you agree to accept service credits if SLAs are not met.

They’re a solved problem in the sense that the tools exist but that still has an ongoing operational cost to correctly setup, secure, monitor and test – and there are plenty of examples of expensive failures or security breaches caused by people thinking it was an easy solved problem.

Deciding whether you get enough benefit from doing that yourself is a classic business trade off which any experienced engineer should consider.