Brummels are not really there to lock the tail they are more like insurance. If the brummel has been loaded it will be upset which means the tail has slipped, that is, the splice has failed and you have been riding on your insurance! In practice with all my splicing I have never come across a brummel that imho had been loaded.
Tails are most vunerable to slippage when load is applied gradually and the outer has not fully bitten on the tail. To cater for this stitch the tail in place. You can also use a throat whipping but that method is not commonly used in kitesurfing.
You can also put an overhand knot in the tail to stop low load slippage but I have never seen this recommended in a splicing manual, presumably because of the loss of strength that results from knots.
For experience it is worth playing with short tails and making them slip under light load then tighten them up and retest with high load, repeat with snatched load etc and you will start to appreciate how it works. There is a certain amount of artistry as well as the basic science! Load a brummell with loosened tail and see how it looks.
It could be argued that brummels are almost pointless compared to stitching but it is a bit like arguing that some insurances are almost pointless! Personally I often use brummels in Line thicker than 2mm but rarely use them in thinner line. I hand stitch tails, and only occasionally use knotted tails and throat whippings.
I liked meri's tip, a good idea for strong short connections.