Theorizing is all well and good, but experimentation is the true test. I swapped my 4 field howitzers for 6 field mortars, both Atomic level, mortars with the library research to increase range to 5. My howitzers have almost max uní skills, my mortars about half. The rest of my army (5 main battle tanks, 6 infantry, 2 tactical helicopters, 39 Royal Green Jackets, 5 mk6 strike fighters) stayed the same. The mortars work as well as, if not better than, the howies.
I am level 237 Cold War Age, and I keep my medals in the 1300 - 1800 range so I have silo-less bases to raid. I will hit any base under level 190 as long as it does not have any defensive blessings, and has a relatively straightforward layout. I use tactics, mercenaries, and alliance troops only as a last resort, or if I come across a juicy base and want to be sure to clear it. I drop tanks, mortars, helicopters, green jackets in that order. I use my fighters right from the start to take out anti tank guns and mortar buildings, after first clearing any buildings on the flight path that shoot at planes. I rarely lose a fighter.
What would happen with howitzers is one would wander off on its own and either roll over a spike trap or trigger an ambush trap and die. So in about 50% of attacks I would lose at least 1 howitzer. The mortars stick together, so they are protected by my green jackets and tanks. They also fire at defending troops, which is actually a desirable behavior because they protect themselves. I rarely lose a field mortar, and even if I do lose one, I don't care, because they are quick(era) and cheap(er) to retrain. I will keep using them and report back in a day or 2 with any additional observations.
The only downside is I no longer have howitzers to donate, which is what most people want. Or maybe that's a good thing