GuiXT vs. ABAP development: If I compare ABAP development with GuiXT development, can you comment on both in terms of the following:

Complexity -
System Performance -
Efforts -
Meeting critical requirement -

When you say "ABAP development" this can mean

(1) Modifying SAP transactions
(2) Building your own transactions

In contrast to (1) or (2), GuiXT changes the SAP UI without changing SAP code. This is not possible with ABAP development.

With the approach (1), it is hard to guarantee that the transaction will still behave correctly, e.g. does not update the database with invalid data. In addition, you need to understand the SAP program logic. With GuiXT you only need to understand the UI functions, but not the implementaiton logic of the transaction, and you are sure that no incorrect or unauthorized database content can occur.

Approach (2), yes, this can be used for relatively simple transactions. The problem here is that the users might need certain functions of the SAP transaction (now or later on) that may be hard to implement in your own coding.  You then run into a situation where you can neither suggest that the users work with the SAP transaction (since it is too complicated) nor can you implement the additional function in your simplified transaction.

With GuiXT you stay in the original SAP transaction. If you delete a menu function, pushbutton or entry field that turns out to be needed later on, you simply comment out the corresponding "del" statement in your script.

The system performance can be better with your own transactions if you succeed in impelementing it better than the SAP standard transaction. But it could also be worse.
With GuiXT you normally need the same system resources as the original SAP transaction, but you simplify the UI.