When you sould NOT use cloud

As an AWS Cloud Architect I quite often hear opinions like "you're working with AWS so you want to move everything there". Definitely not true, and here I'd like to name cases when it's impossible or makes no sense to move to cloud. 

• Rare OSes. HP-UX, Irix, SunOS, you name it. They may be just not available or not supported by hypervisor.
• When you need direct hardware access, for example to USB tokens. Sidenote: in 2019 AWS launched Bare Metal instances which can (at least in theory) do those jobs however it may be too expensive or unsuitable for such tasks. 
• When you need really quick (<50-100 msec) response time. It's about physics: we all are limited by the speed of light. There are solutions which may fit (and may not) depending on your needs:

1. Wavelength Zones are "micro-AZ" in dozens of sites (mainly in US) so if your customers are close to those locations - it may help
2. CloudFront CDN: the edge cache is usually close to the user so users get cacheable objects quick.
3. Lambda@Edge: Lambda runs on edge location to save time. 
4. AWS Outpost: you can use it to run small AWS cloud on-prem, on your hardware.

• Legal restrictions. If you're required by The Law to have all your data in your own facilities etc. However, abovementioned Outpost may help you - depending on price and efforts.
• When you have access to really really really cheap compute power elsewhere and your load level is flat, you don't need any auto-scaling, you don't add new services, don't go serverless or always run the same amount of containers. If you own a datacenter and have free electricity and workforce. 
• The most important reason when you should not use any cloud: when you don't understand what you're doing. It's very easy to set up in a few clicks something that costs e.g. $20/hr and forget about it until you get the bill. At that moment you'll find that month, unfortunately, contains too many hours. 

Let The Cloud Power Be With You! :)