It depends on how close you are to towers and how many are nearby. simulcast is greatly reduced if one of the towers you are picking up overpowers another. This is fairly typical of people in busy simulcast areas or those that do not have a tower close by that they are getting a strong signal from.
Like real estate, location, location, location! Macomb County's simulcast system had 9 sites and they recently added another 4 or 5, not sure at the moment. Most of them are all around me and several of them just about the same distance. I'm not certain why I've been so lucky for about 18 years in this location with just about any scanner. When they first enabled simulcast it wasn't great but as more towers went up the worse it became. However I recall some of the techs discussing the problems being related to a timing issue between sites. Macomb County's radio techs are some of best available and with a lot of tweaking things it suddenly really cleared up and has remained like that for year and years. I use my scanners in my office, with indoor antennas and less seems to work better. One has a Comet Miracle Baby, one is using a Diamond RH77CA, and sometimes I use a Diamond D220R. My BCT15X (not digital) is using a D.P. Productions MilTenna Omni. Life is good!
Some conditions that may be helping is I have good elevation, a 120' long row of long-needle pine trees on one side of my home, a cold air return air duct inside the wall right behind my scanners, a large double-walled steel computer tower 2-1/2' to the right of my scanners and a 4-drawer steel file cabinet in one corner of my office, as well as aluminum mini-blinds closed, about 10' from the front of all my scanners. So I guess you could say I have a selective Faraday Cage!