Sitecore provides different components and modules, and some of them are not ready to be used in a PaaS environment such as: E-mail Experience Manager (EXM) and Sitecore Commerce. For a full list, check this compatibility table: https://kb.sitecore.net/articles/880886
At the time of this post being written, Azure is generally available in 36 regions around the world! However, there’s a possibility that either Sitecore PaaS, prerequisites and/or additional tools aren't available in your region of interest and as suggested by Sitecore, "you can in most cases swap out a service for an IaaS equivalent". So, prior to deployment, I’d recommend that you have a look at Jason St-Cyr's original cheatsheet and get a quick overview.
Sometimes understanding license models can be a challenge. The role of PaaS is to free customers from being responsible for the product layer and to take ownership of it. Nevertheless, during Sitecore PaaS provisioning a license file is required, and we encourage you to contact Sitecore directly for further questions.
These are the main factors to consider beyond the cost of the Sitecore PaaS on Azure. It’s likely that if you dig further, you will discover even more.
In conclusion, the decision to choose PaaS may be appropriate for you, but first, we recommend that you pay attention to the factors listed above before purchasing.
If you have questions, do not hesitate in contact us! or dive into more of our insights.
 Region Availability - https://azure.microsoft.com/en-us/regions/
 Azure data center compatibility table - https://kb.sitecore.net/articles/617478
 Jason's cheatsheet Azure services availability -https://www.valtech.com/globalassets/00-global/02-images/04-insights/sitecore-paas-on-azure-is-it-all-about-cost/sitecore-paas-regions-table---new.png
 Security - http://www.nonlinearcreations.com/Digital/how-we-think/articles/2017/04/Sitecore-Security-in-Azure.aspx