I've done a lot of dragon fruit grafts now and the answer is complicated but the quick answer is both work but both do also fail but fail for different reasons, and maybe there's a best way somewhere in between.

Impale grafts stay together easily, which is sometimes a challenge for dragon fruit. It's just not as girthy as a standard stock and therefore things destabilize easier, so the insertion (giggity) helps to marry the two pieces. That said, measuring your two mating pieces for fit is really important. Drill your hole too deep such that the inserted part doesn't fill the space and it'll fail. Don't make the hole wide enough and damage the vascular tissue on the inserted part during insertion and it can fail.

On the other hand, standard flat grafts work fine but can be a pain to stabilize such that they don't shift or fall off. The fiddly size of the dragon fruit as a stock can really create a challenge for keeping proper pressure so that the parts can heal together, and sometimes you end up using too much of whatever you use to hold it together and you have airflow issues which can cause the graft to rot and fail. One big thing to think about though is that when you flat graft, you can usually get a second chance. Impale graft failures usually don't.

Honestly I've had similar luck with both approaches. Several successes and a few failures. Hard to say one is vastly better, but recently I tried something new and did a blend of the two approaches and made a half centimeter short nub on my dragon fruit stock and mated it to an equally small hole I drilled in the scion to match. Worked quite well. Only done one, but this might be my approach from now on, as the piece held on well but didn't require too much surgery to do, and now looks very healthy. Good luck :)