There is without any question that the single best radio communication interoperability is plain old in the clear wide band analog simplex radio to radio communication. Once you co-mingle various protocols, trunk system to trunk system, trunk system to conventional, encrypted to clear, digital to analog or if you have to go through or rely on a dispatcher for IMMEDIATE emergency communications or what ever combination you can think of you are asking for problems. The very last thing that the comms guys wants to hear from any command staff or incident commander at a event of any kind is " hey,,, can you patch our radios to theirs" They all think that that all those magic boxes that were purchased after 9/11 from companies that now employ the retired cops that they used to work with will do everything they were told they will do.
The SPEN radio system or I should say the way the system is utilized( not a trunk system) is the best form of interoperability anywhere in the country, A simplex VHF in the clear radio to radio will only fail if one of your radios is broke. one of the biggest downfalls and shortcomings of SPEN is the politics that I have seen first hand, Any dispatchers or PST's here will probably agree with this but in the cases I'm aware of the dispatchers are so understaffed, dangerously understaffed that they have to pay attention to the personnel in their respective departments that they are responsible for and the volume on SPEN is lowered,
for your question on TG's you run into the above mentioned issues on different system protocols. When a cop has an emergency and he picks up a mic to call out something he wants it instantaneously and with the number of VHF secondary radios in many cars some other close by cop might hear him, If he is using a TG assigned to a certain county/state system the other cop might not have that ability, keep in mind it might not even another cop that hears the call, It could be a fire truck, ambulance or even a scanner listener who can pass the info on.
another factor is cost, a good VHF conventional radio could be had for a few hundred bucks and you NEVER get a busy or out of range bonk.