Reasons to keep site on-premises

[Dean Gross]

Background - One of my clients is moving their data center to Azure and is considering splitting the existing SP 2013 farm into a 2016 farm in IAAS Azure and SPOnline (hybrid config). We need to decide which sites can get moved to SPOnline and which ones must stay in SP 2016. They currently have many sites with using Excel Services (which is not available in Azure (because Office Online Server can't be licensed there) so I'm thinking that they will need to go to SPO or that they will need to do some new work with Power BI Report Server. They are also using a lot Nintex workflows - which may or may not be movable to SPOnline (we are still investigating them) . 

My question - what reasons are other organizations using to keep SP Sites on-premises?

[#2]

The three reasons I see most:

- Trust / privacy - companies (think they) are not allowed to store certain data in the cloud
- Customisations - heavy and complex customisations that support (business critical) processes, and that cannot be brought to SPOnline without a lot of rework

- Politics - IT people / departments afraid to lose their job when moving to the cloud

[#3]

In addition to what you've already mentioned I've seen some cases where sites were customized a lot using server-side custom developed solutions making use of SharePoint's server API's or other API's for integration with different on-premises products. These solutions are not directly usable in SharePoint Online so they have to be rebuild or replaced first in a SPO-compatible way. Such an effort can be very costly and might be a reason to keep sites on-premises for the time being...