Summary: This post will describe all things to keep in mind when configuring Hybrid Taxonomy.
Microsoft released Hybrid Taxonomy in preview for SharePoint 2013 and SharePoint 2016 on-premises. This makes it possible to replicate terms between your on-premises SharePoint farm and SharePoint Online. For more information on hybrid taxonomy, see https://support.office.com/en-us/article/Configure-hybrid-SharePoint-taxonomy-Preview-dd287a75-09e0-403e-974e-4cc84a152815
1. Term limit
In SharePoint on-premises (2010, 2013 and 2016) the maximum number of items in a term store is 1.000.000. For more information, see: https://technet.microsoft.com/en-us/library/cc262787.aspx#termstore
In SharePoint Online, the maximum numbers in your term store is only 200.000.
This means that you can only have a maximum of 200.000 terms in your on-premises Managed Metadata Service Application if you are planning to use hybrid taxonomy. Take this into consideration when you configure hybrid taxonomy.
Hybrid taxonomy is currently still in preview. Keep in mind that things might change along the way.
If I find any more considerations I will update this post.