The plus of the ultimak and MI rail are that they are barrel clamp designs which guarantee that the sight is perfectly aligned over the bore.
Anything mounted in the back (on top of the receiver, or the side rail) doesn't proceed under that assumption because it depends on how the gun was built.
There can be a lot of variance there.
If the RSB isn't pinned perfectly square, the trunnion isn't sitting perfectly square in the end of the receiver AND centered within it, etc. If your top cover is tighter on one side than the other or the rear sight leaf pin holes are not drilled square through it, the rail will want to sit leaning to one side or the other. If your top cover is tighter on one side than the other, you've got this issue and should go with the ulti or the MI rail.
I've got examples of them all, but the only ones straight enough to handle an AKARS or TWS are an arsenal and a krebs. The safest best is the ultimak or the MI handguard system. Those will work on every gun, every time. Don't add a side rail. It's a waste of time.
HTH,
--Fargo007