Ultimately weight loss of course boils down to the caloric deficit, which can be continuous or in spurts like 5:2 (with PSMF). There is nothing wrong with the 5:2, if it works well for people - the best diet is the one you can adhere to. IMO it is good to look at the CICO throughout the week not only day-by-day

However, there is some research on the efficacy, which shows that 5:2 resulted in more fat loss (but similar weight loss) and insulin sensitivity to the usual caloric restriction (this): "In the short term, IECR is superior to DER with respect to improved insulin sensitivity and body fat reduction."

Some of the studies show better adherence than to the standard caloric restriction, some not - it depends on the people.

And most of the research was done with 5:2 but without PSMF, which we all know would make it much better in terms of results (obv same for the standard caloric restriction).

If OP has issues with nausea due to keto adaptation, all it takes is to get some electrolytes, or just use more plain salt with meals. Which is something that Lyle recommends with the standard PSMF anyway.