There are quite a few threads on this - is would link them in here, but I'm on my phone so it's quite hard.

Ive also booked and flown these kinds of tickets.

My main takeaways, as there are pros and cons;
-In case of schedule changes or cancelations you'll have to deal with IB, which is considerably less easy that QR
-My FCO-DOH flight was axed (when QR loaded the new winter schedule) ; IB forced me to fly FCO-MAD-DOH because no other flights were available that day and they wouldn't let me fly on another day (extra Qpoints and avois though + MAD had q suites and FCO not, which was nice)
-There were issues with my returnflight DOH-FCO too, and IB forced me to fly their own metal via MAD (which was far from ideal, added an extra stop, but yielded extra qpoints and avios which pushed me to silver with just this one trip, lol)
-Lounge access in DOH and outstation IS included compared to the cheap fares QR sells
-if your flights have an IB flight number, but are QR operated, they will not count towards the 20%/4segements needed for status renewal/accural.

As you see there are some pros and cons.

If the saving is this significant though, I'd book with IB again.