Cloud is also good for episodic use of expensive exotic systems like HPC and GPU fleets, if you don’t need them all the time- I call this serial multi-tenancy.
Cloud is not economical for massive storage, especially if you’re not willing to use backup solutions and reduced availability. For example, AWS S3 default keeps multiple copies of uploaded data; this is not comparable to typical on-premises RAID 1 or RAID 3. You can save money with reduced redundancy storage but then you have to take on more of the reliability burden. Likewise compute is cheap if you’re buying multi-tenant instances, but if you want dedicated instances or bare metal, then the economics aren’t nearly as attractive.
Cloud is also good for experimentation and rapid development - it’s so much faster to click a few buttons than to go through the hardware acquisition processes at many enterprises.
The companies that regret cloud due to financial concerns usually make two mistakes.
First, as noted above, they pay for premium services that are not directly comparable to on-prem, or they use workloads in cloud that are not cloud economical, or both.
Second, they don’t constrain random usage enough. It is super easy for a developer doing some testing to spin up thousands of dollars of bill. And it’s even worse if they leave it at the end of the day and go home- it’s still racking up hourly usage. And it’s downright ugly if they forget it and move on to something else. You have to be super disciplined to not spin up more than you need and turn it off as soon as you’re done with it.