We will ask Adobe which metrics can be combined with the dimensions you have selected when connecting, and what metrics you have permission to. Every metric supported by Adobe for your report suite and that we support will be downloaded, including custom events.
Metrics that we don't support include unique metrics, lifetime metrics and calculated metrics. Some calculated metrics are automatically setup in Funnel, others have to be recreated.
The maximum number of elements you can select at once is 4! This is a limitation in the Adobe API. You can add multiple connections with different breakdowns, but remember that the metric values will be doubled if you look at both reports at once.
There are two branches of elements, Commerce and Traffic. Elements from those two branches can not be combined. There are also what we call Shared Elements which can be combined with either Commerce or Traffic elements.
Only select the elements you will actually need, since more dimensions increase the complexity exponentially. This might lead to reports taking longer time and even failing.
WARNING If you select an element that is not supported by your report suite that element will simply not show up in Funnel.