When doing risk assessments you should always face the problem with the solution.
Side mounting is a partially redundant solution, which is usually adopted as the response to a problem.
The problem is some sort of higher than normal requirements, as related to diving deep, with deco or in overhead environments.
So we are already close or crossing the border of tech diving, which is assumed to be a risky activity.
So, if side mounting is associated with these high-risk diving profiles, generally it should be concluded that the solution adopted mitigates these risks, but they remain inherently larger than diving with low-risk profiles, where sidemounting is generally not practiced, nor needed.
Of course, if side mounting is used also for normal, low risk, recreational dives (for which it is unneeded), then of course it adds redundancy, making the dive safer.