The compression fittings that I am talking about have a metal keeper ring inside of them. The towns that do allow them justify it by saying that if anyone ever digs through them, they will pull the copper out of the corp, and not the corp out of the main. Then again, there are other towns that don't like them because they don't trust them. Personally after having assembled probably 1000's of these things, getting 2" flare copper tight underground, in a ditch, with no room to work, tight enough not to leak, is not an easy task. Flaring 2" is not an easy task. Compression is much easier, but you need to see if the town will be ok with it. I don't care about what "most areas" allow, I am telling you to find out what YOUR AREA allows and giving you my input on what is more user friendly. MOST towns think that they have it all figured out, and that they know everything about everything, so it doesn't matter what we think, it is what they think.
I go from one town using no shear mission couplings because they are great, and another town doesn't like them. Another town likes to cut out a section of sewer and cut in a tee, another will allow an insert a tee to be cored in. Chimney seals on manholes in one town, not necessary in another. Some towns will go right along with doing a pressure tap through a water main valve, others want a tee cut in. Some will require a corp be directly tapped into the water main, others want a saddle. EVERY one of the inspectors in these towns will look at you like a donkey for even suggesting that there is another way, even if the circumstances would warrant the other one.
I just love the towns that don't allow couplings at all. They think that you can get 2" copper in continuous rolls, good luck finding it in anything longer than 60' lengths. Even if they did make it, which last I checked they don't, it is bad enough rolling out as it is.