I recall reading in the past that mounting optics such as ACOGs , Aimpoints and EoTechs are a problem due to lack of mounts that were made.
I am looking at both a AR and a ARM in 5.56mm.
What are my options?

For an ACOG you can use the AR15 "channel" type mount.

If you wanted to mount an Aimpoint or Eotech, you could use the same thing with a picatinny rail mounted in the channel, but the optic would be excessively high and you wouldn't have any cheek weld.

IMI made a picatinny mount that replaces the gas tube. I used to have one on my Valmet, it works but it makes disassembling the rifle a little harder. Biggest problem with it is that you have to have the optic mounted so far forward.