The managed metadata support within PowerApps is useful, but there are two massive limitations at present, which must be effecting many businesses in their attempts to use managed metadata columns within PowerApps.
The first limitation is the missing tag icon for displaying a termset in a hierarchical presentation. Users familiar with native SharePoint functionality can access this tag icon in both classic and modern SharePoint, but it is missing from PowerApps. A hierarchy representation of a termset is useful for users so they can see the options available especially for complex or large termsets. This leads onto the second and most limiting issue.
The second limitation is the limit of 20 terms displayed within the combobox field of a managed metadata column. This limitation has been the blocker for any kind of workaround I have attempted. Users understand that they can type in the term (using type ahead), but only 20 terms are displayed when they click on the drop down (without using type ahead). This causes confusion as users think there is only 20 entries to pick from (there may be many more). This is especially true if the user doesn't know the full set of terms (and they shouldn't need to).
I must admit, I'm surprised the this lack of support, there must be many businesses asking for this, or have decided that SharePoint integration for PowerApps isn't ready.