Rounding of the back or too high of an arc are both going to be bad for your back. Ideally you want to aim for a tight, straight back. The mechanics of a proper deadlift are different than those of an atlas stone lift, good mornings, etc. The bar placement will determine the particular mechanics of the movement. Rounding of the back happens because of a weakness along the chain or a deficiency in form. You'll notice in a lot of big lifters when they go higher their backs will arc slightly simply because of the massive load they need to lift, usually when you see videos of powerlifters aiming for a high weight it's at the end of a training program and most likely a weight that is some measure of a percent over their predicted 1rm, then depending on the lifter they go 5-20 pounds over that even. So a lot of lifters in 1rm videos or even double or triples with slightly rounded backs probably haven't even lifted that weight in the gym before.
Second a breakdown in form can cause the rounding of a lower back. If the hips rise too fast from the initial portion of the lift it will cause the rounding of the back. This is because a lack of keeping the hamstrings tight and a proper arc in the back as well as just not having practiced the actual lift enough times to keep proper form. Long term effects of rounding your back is inevitable damage on your back especially as the weights get heavier. It's not an efficient way to move the weight, the rounding of your back requires that your back muscles shift from a stabilization focus to actually needing to pull your back straight.
Here's some videos to illustrate.
Hips rise first out of sync with the upper body, there is absolutely 0 pull on the bar until he fully raises his hips all the way up as a result the lower back rounds out, as he rises to compete the lift the back muscles need to straiten out the back to get it in an upright position to complete the lift.
Versus someone using much heavier weight
You see Derek Poundstone here, I could've posted a video of any good deadlifter, but he moves in a very deliberate full body motion even with that tremendous weight.
Hoped this helped.