Just took a few minutes and looking at the way they rate films , the ratings are very subjective it seems. Even in the last 5 years of releases they have different descriptive phrasing for their rating reasoning, it's not very consistent.
Something could have insane, over the top gore but I could understand a 15 rating but then I see they gave 'The Coffee Table' a 15 and I for certain would personally put it at an 18 (DO NOT READ if you ever plan on watching it)
Spoiler
with how realistic of a scenario it is and the dread of it all, a man falling apart as he deals with his baby that has been torn to pieces by glass, the head he can't bring himself to pick up as he tries to hide what happened from his wife returning from the store, and the tension from her being so happy to finally have a child in her 40's and then this shit happens, she's gonna find out, it's all very real the way it is shot
I imagine Romulus will be impactful gore and, and like someone mentioned without sexual violence it's probably not gonna to merit an 18 these days based on what I've seen just going through their ratings I've the last 5 years.