The risk of an aircraft swap is there with any airline. With BA the risk is that you'll get one of the fleet dogs with a filthy cabin.

I've been lucky with QR so far, the only time I've had a slopey seat is when i've booked an A330 flight. They're fine on a day flight as the rest of the product is so much better, but for sleeping i agree no angled seat is ideal. They have almost 100 A350s and 787s on the order book though so the remaining A330s aren't going to be around for much longer. My only issue with QR is the feeling i get from them that if something goes wrong on my booking im going to end up on the back row of economy on a flight two days later than planned.

I have to say though - you cant book a flight on a month old aircraft of which only one is currently in operation and complain if it gets subbed.