You can view all data in Management data warehouse for all instances that upload collected data to the warehouse database using "Management Datawarehouse overview report"
You can view this report by following these steps
1) Open SQL Server management Studio, Connect to SQL Server instance that is hosting Management data warehouse database (MDW)
2) Right click on MDW database, Select Menu
Reports -> Management DataWarehouse -> Management Datawarehouse overview
3) On right pane you will see a report that list all the instances that upload collected data to this MDW.
for each instance, you can launch the hyperlinks for reports that lets you view data collected by system collection sets