Hilary Curtis,
Is your response referring to hosted feature layers within a Portal Data Store, or feature services published from ArcGIS Server to a federated Portal?
I agree with Kevin MacLeod that polygon or line feature services published from ArcGIS Server are just too slow to draw to support being used within a web map. However, hosted feature layers published to the Portal Data Store are incredibly responsive.
We are a medium sized government agency that supports a pretty good number of GIS users, and if would be amazing if we could just publish feature services from ArcGIS Server and have them available for our users to use in web maps. Most of our users are not heavy editors that need Pro or Desktop, and being able to provide them web maps and feature layers as a way to construct maps, is one of our desired goals...but until published feature layers from Server perform better, that's not a reality.
Before someone mentions alternatives...like caching, or publishing data to a hosted feature layer...that's all well and good, but it requires maintenance, and perhaps a lot of it if you have several or tens of hosted feature layers you want your users to be able to use. Re-building cache takes time and a lot server resources. Overwriting hosted feature layers takes time and both of these options means your data is a snapshot in time until you update it.