It's true, sharing services that use transportation (garbage, recycling, police, fire) can be hit and miss. The biggest risk is that the vehicles get stuck in regional traffic, so they don't arrive in time to start and can "forget" why they're there in the first place. It's unfortunate, but the regional sharing has always been difficult to impossible to balance well because of traffic.
Since the player can't add or enlarge regional highways (without mods), there isn't a lot that one can do other than making sure the city streets are flowing as well as possible. I gave up trying to share services that rely on transportation due to the issues. It's annoying, for sure, but now I just give each city their own garbage, recycling, education, etc. The sewer, power and water shares all work well. I usually create one city in the region that is the "utility" city and its only function is to supply the region with those three things. Frees up some room for the other stuff.
Since the player can't add or enlarge regional highways (without mods), there isn't a lot that one can do other than making sure the city streets are flowing as well as possible. I gave up trying to share services that rely on transportation due to the issues. It's annoying, for sure, but now I just give each city their own garbage, recycling, education, etc. The sewer, power and water shares all work well. I usually create one city in the region that is the "utility" city and its only function is to supply the region with those three things. Frees up some room for the other stuff.