The flying unit will ONLY be barbecued before it can act if it's of low level OR if the enemy target has multiple heroes/ranged units.
You are the underdog, by definition you are weaker than the other player. So probably you just can't afford to suicide expensive fliers to try to send one or two spells when all races have a level 1 archer unit. And if they are cheap fliers, they are going to get barbacued. And probably the enemy will heading the assault with a hero, so he will be able to cast at least one spell (or two if the hero is a caster) and kill even a strong flier.
Your single sucide flier is going to get barbacued 9 out of 10 times, no matter how you try to look at it.
And I never said a single unit with spells would kill them... I wrote they would be wounded upon finally reaching a town... and they would. And the heroes only have a single hex for a domain so there's very little they can do as compared to what a wizards_tower and/or magic relay.
Lots of things can heal units on the map. And a single hex for domain is all that a stack needs to get buffs, summons with reinforcements, heals from the mage, so in fact, with a signle hex you can do mostly everything. Probably the only "useful" spells that you can use right away would be the Divine Storms and things like that, but if you build an army inmune to them you can even cast them upon yourself and that's it.
Of course the defender gets first action in battle... this is just another method how the underdog receives an advantage. On this same note, maps can exist where towers must be passed to enter a players section of map... this is another example which provides the underdog defenses.
That's an advantage to the defender, not to the underdog, don't mix both things. For example, that allows the power player to leave cheap units in his towns and use the strong ones to move forward and attack, that doesn't help the underdog at all.
Think what would happen to an approaching enemy army which has to attack two towers in your domain to eventually attack your city. The domain is also very useful for domain spells such as the fire domain and poison domain. So the domain is clearly valuable for underdogs, in fact you only pointed out another underdog defense which is how the battles allow the defender to go first.
Domains overlap, the attacker can expand his domain building towns, with heroes, improving his towers,... So most times he is going to have his domain with him helping too. And if he doesn't and is really that important, he can just build a town, put a tower there and problem solved.
I don't see how this response is even remotely related to this discussion of walls and defenses with magic. Please stay focused on the current topic of discussion or send me a PM if you wish to change the topic.
This answer is related to this thread. The thing is that in a "magic" world, there are lots of things that don't make so much sense as in a "non-magic" world. For example, big castle walls.
I have never said that defenses aren't useful, I have said that normal world defenses are mostly useless on a magic world as there are lots of ways to avoid/destroy them and if you start adding insects/undeads,... they just look plain idiotic, and that breaks the inmersion on the game (at least for me, probably not for you).