I play 2 advanced human cities and I also don't think Mortars are as bad as some make them out to be. I have won many fights with them. But they are not great either. I could have won those same fights using Orc Strats and would have taken fewer losses. Essentially I chose to use Mortars in easier fights to preserve my supply of Orc Strats for harder fights. The main issue with Mortars is they have long range but a relatively low attack rating. Furthermore, their special ability does not help them do any extra damage. You mentioned Fire Phoenix in your post. The attack bonus from Fire Phoenix really helps Mortars. Without this bonus they truly suck. They really need the extra +50% damage from the Fire Phoenix to perform. The Temple of Toads AW will also help, since it gives a permanent attack bonus to all your heavy range troops. All that being said, 3 star Orc Strats or 3 star Toads are still going to outperform 3 star Mortars in virtually every situation. However in the mid-game you have 3 star Mortars and don't have 3 star Strats or Frogs yet.
Thats the entire key/
I am looking forward to the frogs for sure! I just opened chapter 7. Right now it's 2 star mortars and 1 star Strats. My phoenix is at +35%. I do combine my mortars with Strats if there are mist walkers out. It also comes down to terrain. Mortars behind a lot of obstacles will create a lot of problems for the other side. And yes, I have my eye on the temple of toads when the time comes!
Frogs arent much better, its the frog prince that's a lot better.
Both the frog and the mortar have the same issue, there damage output is very limited but they have a huge range.
The main issue is that the maps arent big enough for the range to be effective, the enemy is simply too close to you to quickly to really benefit from it (unless the map is just right).
This is why mortars loose battles very early compared to orc strats and golems.
The main difference between mortars and frogs are the special 3* power.
Mortars limit the enemies attack, not very usefull because you do not want to get into combat with your enemy. this is why they suck, the enemy alwaysgets a chance to hit and hurt you.
Frog prince has the special power lower defence, this means the enemy dies quicker. this makes the difference between the two day and night.
The first hit is as rubbish as a mortar but the second hit hits hard (tho with the recent combat changes not as much as before)
Golems and strats have more bulk, they lack range, but they hit hard and have many HP to catch the brunt if they get hit.
When you once played elves and are used to that power, and then get a human account and use mortars you just get shocked about how bad they are. you start loosing battles way way way way way before you start loosing them with golems.