There are a number of discussions on this if you search “Red Dot Sight Mounting” or similar words.
The information above on the Sig is good advice. The Romeos are one of the least robust sights on the market.
Any factory milling for multiple optics has downsides compared to custom milling for a specific optic. What you are willing to spend and your intended use of the gun would be helpful to know. A target gun does not need the same robust qualities as a fighting gun. Lower cost and easier alternatives, like a CORE, probably work OK for target use and gun games. If something breaks in use, no big deal.
Choices and compromises don’t stop there, and you seem to know that from your original post. Optics run from cheap and almost unserviceable to expensive and very robust. They have different footprints and different heights, thus requiring different height BUIS to run properly with a lower 25% co-witness.
Any time you mill a slide and then add a mounting plate for a certain optic, you raise that optic above the slide. Some plates are better than others at holding the optic. If the plate has fore & aft stops as well as the mounting screws, you get a fairly decent mount, but all the screws will be shorter and less secure than mounting a RDS directly into a custom milled slide. Short screws may break. Long screws not. If only screws are holding the sight on, and they are short, I wouldn’t spend my money there.
As I understand, the CORE suppressor height sights are too low to co-witness with some sights mounted on CORE plates, requiring that you buy taller sights.
The aforementioned problem with the 2.0 Loaded Chamber Indicator (LCI) is another consideration. You don’t want to leave the slot unfilled. Blank inserts may have attachment points far enough to the rear that milling for a long optic is not possible. I don’t know for sure but I would just avoid the whole concept.
I believe S&W will provide a 2.0 CORE at some point. I don’t think it is worth waiting for. I don’t think it is worth custom milling a 2.0 with an LCI.
My M&P 1.0 FS 9 milled for an RMR with Heine Straight Eight raised height night sights and Apex parts runs & shoots wonderfully. I also have more money in it than many people are willing to spend. It is my EDC so it is worth it to me.
I have a Shield set up the same way with the Shield RMS that has a narrower footprint. It also is reliable and shoots great.
I have a G19 custom milled with an RMR 06 and custom parts from Suarez International. Also expensive and great running/shooting gun. I like my M&P better.
I don’t like the Springfields as fighting guns. I am one unimpressed with Sigs. Glocks are OK, but fat gripped and I do have to remember to point them differently than my S&W’s. However, the M&P 2.0 cannot shoot any better than my customized 1.0, so I’m not looking to change anything.
