At first, it takes a little additional effort when PTFE line and nuts are not in place (that is, not properly aligned). Once done, this doesn't take more effort than to push the filament through regular bowden tube. The "curve enforcer" can help if the extra PTFE line is a little short, but with 20 cm, it shouldn't be needed. Adding some silicon oil (other mention canola oil) in the PTFE does help to smooth out, but in general, the added benefit of the double traction overcomes largely the little added friction. Printing semi-flexible filament is perfectly possible and with great results:
For fixing of the coil, the trickiest part of this build, having it only glued on the stepper shaft is not enough, it must be compressed axially, otherwise, it will "uncoil" and break on pushing or retracting the filament (depending on the direction of the filament vs the direction of the coil) and this will happen:
Some alternatives were available (Clipper lighter with solid flint wheel and internal 5 mm diameter), but more recent models have smaller holes and drilling this stuff is REALLY hard (even with diamond drill, the diamonds get loose!). The best is to tap the axle with an M3 thread and use wasted bicycle chain rolls as spacers (ask them to your local bike shop, they should give them for free).
Steps to tap the shaft of the stepper motor:
You should be good to go... Hope it helps. Like I said, not your mother's extruder! But it worth every cents...