Union station capacity is likely the main issue. In order to relief the core subway system, a new RER tunnel through downtown is likely needed.
The rail corridor width through East Toronto is another concern. Two more issues are the grade separation of Scarborough Junction, and the width of Uxbridge sub corridor south of Eglinton.
Hopefully with the Metrolinx Relief Study underway and the accelerated capacity crunch at Union with the implementation of GO RER, they will come to the logical conclusion that building the DRL as part of the GO RER network (in essence routing SmartTrack through a downtown tunnel instead of through Union) makes more sense than building the DRL as part of the TTC network. The TTC study however will undoubtedly conclude that the DRL needs to be (surprise!) a TTC subway.
Building the DRL as a GO RER line, or as the local component of the GO RER network, kills 2 birds with one stone: it provides the subway relief that is needed (both by offering a convenient transfer point for subway passengers, and by offering new suburban branches to allow Toronto passengers to avoid the TTC subway network entirely), and it eases the Union capacity crunch by routing a significant percentage of the total RER trains through a downtown tunnel instead of through Union, while still providing the same network capacity to the CBD.