Known existing integration (white list):

Feature4.x nameNotes
Content Model 5.x aimed at not adding any features in the content/data model to make sure no breaks happened in 4.x, new features will instead be introduced as we approach 6.x with new storage engines that can support them.
Content(Content) Object

 

Content RelationsObject Relations 
Field(Content) Attribute 
LocationsNode

 

Section 

 

ContentType(Content) Class 
FieldDefinition(Content) Class Attribute 
FieldGroups(Content) Class Attribute Grouping 
ObjectStateObjectState 
UrlAlias  
UrlWildcard  
Language  
User  
UserGroup Can by design (and intention) only have one location in 5.x
Policy  
Policy Limitation  
Role  
Role Limitation  
Role Assignment Currently not possible to delete specific assignment by id
   
Web stack 5.x web stack is Symfony with several 4.x features on top
ControllersModules 
BundlesExtensions 
View ProviderOverride systemRestricted to viewLocation in 5.0 and additionally viewContent in 5.1.
Theme systemDesign systemPlanned for 5.2, but not in yet!
HTTP CacheView- / Static- CacheFor anonymous users in 5.0/5.1, for all users using "userHash" in 5.2.
SiteAccess  
Site(Access)Groupingn/aNew in 5.x
   
Data  
Persistence cacheCluster- / Class- / ClassAttribute- / "Global"- cacheIntroduced in 5.1, currently missing support for multi repository (multi-db) setups
IOFile ClusteringCurrently uses 4.x cluster system in the backend, new and simpler file clustering is possible when new storage engine is created in the future
   

Known missing integration (black list):

FeatureUnsupported
RatingUser Rating not currently supported by API, planned to be implemented later when some convention for FieldType provided API's are established
ShopNo shop or field(/data) types related to shop currently supported by API or planned to be supported by API (plan is to provide general integration points for use with other (Symfony based) shopping systems)
Commentezcomments is not supported by API, and is not planned to be supported either (planned to be replaced by disqus integration, can also be solved on customer projects using content model)
Content EditingContent editing module is not implemented yet, so currently own raw editing support needs to be implemented on top of REST/Public API directly.
Module PermissionsController / Bundle Permissions is not integrated with eZ Publish permission system yet, currently standard Symfony firewall rules need to be used.
UrlAlias RoutingUrlAlias has the same model as before and hence does not support routing directly to a bundle:controller:action.