What are the capability differences between implementing the WebSDK client-side and utilizing Event forwarding compared to using the Server Side API for things like data collection and/or deployment of 3rd Party Tags?
Are there any differences specifically related to things like 3rd Party Cookie Deprecation (will one method protect you better from this), Safari / Apple ITP, and setting FPID that are enabled by one method or another?
Firstly, for tracking to Adobe products (Analytics, Audience Manager, Target, Campaign), you'd need to implement client-side Web SDK and corresponding datastreams with the included Adobe services. Event Forwarding doesn't apply for Adobe products.
For 3rd party tracking, Event Forwarding might help with more accurate measurement by circumventing some browser restrictions, though it still depends on Web SDK to send data that can only be collected from the client side, e.g. page URL, data in your data layer, etc. Personally, I won't assume that using Event Forwarding would result in full 100% coverage of all site traffic, because there could still be some technical limitations that prevent such full coverage.