You'll see that we have the same opinion i think: first of all, i did not mean to say that Israel is able to knock Romero down or out because he hits as hard as Costa, but because of his accuracy, as a sharp strike that lands perfectly is more than enough to knock someone down.
Now regarding Costa: i basically agree with you here. The thing is, that Costa hits hard, but i don't think that he's naturally a particularly hard puncher, moreso that his size alone makes his punches dangerous, whereas with a guy like Gastelum for example, you have someone who many describe at undersized at 185 but still has the power to immediately shut the lights out of almost anyone.
I think the best asset of Costa is the fact that he can sustain his unquestionably good power for a sustained period of time/for a rather high number of strikes, hence his great ability to walk people down.
I just think that many people, especially outside of sherdog see him as someone who knocks people stiff with single punches, instead of the relentless grinder that he is imho.