In the past it has been harmful and helpful.
Harmful because it can send your I-130 to the back of the line. If filed when most people choose to.
Helpful because it can pull an overdue I-130 out of the dark, if you're way over processing.
Won't hurt it, as mentioned though so far it hasn't been seen to help IOE cases. Many with IOE have submitted I129f for K3 but haven't heard anything yet, but are also still within the I129f processing time, so I suppose we will all just have to see what happens once those people's I129fs hit USCIS' posted processing time for Nebraska I129fs (which is around 7mo I think right now) which is no 1-2mo as it was with previous I129fs but hey.. 7 months is better than 12+. But that is something that has yet to be seen. No harm in trying, though
There is no harm in filing the I-129F for a K-3.
There is no evidence of there being any benefit at this time either. IOE cases have not seen the suspected bump in processing that some I-130 Nebraska filers in 2018/2019 had.
USCIS policy is to send the I-129F for a K-3 to the same service center as the underlying I-130.
They will generally adjudicate them together. What actually happens "recently" is they will deny the I-129F due to the benefit being obtained via another method - the other method being the I-130 being approved.
If they somehow don't catch it at USCIS, NVC's policy is that if an I-129F for a K-3 arrives at the same time or after an I-130, they will close the I-129F case there.
If it somehow even got past that, consulates tend not to issue K-3 visas.
Only a few dozen K-3s made it that far all last reported fiscal year, and I believe they had a final refusal rate of around 60%.
Last year had only 5 K-3s issued worldwide, of which 4 were issued by CDJ (Mexico).
tldr: No drawback, but no benefit either. Don't expect the K-3 to be issued.
