What exact features are you looking for that are missing?
The primary features we hear the most requests are analysis and image upload for symbols which are planned for the next release. Related records was another one but we just did an initial release to support this and have more planned for the coming releases.
The full parity is not there yet but we are addressing missing features each release. The potential for hiccups may come up as you try to use some of the new functionality in apps built off of older JSAPI. Map Viewer Classic and Web AppBuilder are the primary used on the older API version.
Dashboards for instance retired the older JSAPI version and the main version of Dashboards is built with the same underlying API version as Map Viewer and they have the same parity of features. https://www.esri.com/arcgis-blog/products/ops-dashboard/announcements/dashboards-classic-retiring/
Transitioning from Web AppBuilder to Instant Apps and Experience Builder will also reduce any experience with features being dropped.
We are not back porting any of the new functionality to Map Viewer Classic. Examples where you would see features being dropped when opening a web map authored in Map Viewer in Map Viewer Classic or WAB:
- Popup elements are not supported in Map Viewer Classic but are authored in Map Viewer and in ArcGIS Pro
- Symbols published from web styles, vector symbols, hatch and marker filled symbols
- Group Layers
- Effects and Blend modes
- Aggregation
Hope this helps
Russ