I find defenses very good, especially if they're just a natural part of a creature's stats (can't be dissolved/dispelled). Your mage should probably get some armor on first, but in builds that build up a bit before going on offense, they can be invaluable. Even though there are answers for defenses, the opponent still has to spend mana, if not mana and a QC/action, on doing something that isn't increasing their dice count, which likely means more turns and more advantage for you. And if you have more answers for their Unavoidable than they have cards that give Unavoidable, they're in deep trouble if your mage isn't close to dead.
Also, you can use the defenses that can block ranged attacks to protect your armor from Acid Ball (if your build is very defensive), which is something I don't see most players do even though giving ranged attacks Unavoidable is significantly more difficult.
I'm not sure how well a defense would translate to a conjuration, even though conjurations will probably have them in the future. Right now only the Intercept creatures and I guess walls can give conjurations a pseudo-defense. Being able to give epic conjurations like Spawnpoints a defense feels off though, so going non-epic with the target line should be considered.
The problem is that it's really difficult to have more answers to their unavoidable than they have cards that give unavoidable. Forcefield is Forcemaster only. Helm of Fear is warlock only. Jinx gives them their card back and doesn't work against non-spell attacks. Fumble gets discarded after use. There are currently only four sources of intercept in the game, and they all cost a full action and at least 11 mana to cast.
Every flameblast costs 5 mana and quick action. 6 flameblasts costs 30 mana. 6 fumbles used against flameblasts costs either 18 mana (if thoughtspore is the flameblast-caster), 24 mana (if sersyrix is the flame-blast caster) or 42 mana (if the flameblast-caster is the mage)
6 jinxes costs 18 mana in total, but it refunds both the flameblast cards and the mana spent on them, and requires perfect prediction of the flameblasts.
All currently existing intercept creatures can only stop 1 flameblast per turn, and they all cost at least 11 mana to summon.
Forcefield and helm of fear are only forcemaster only and warlock only respectively.
Falcon precision is relatively easier to counter by destroying the enchantment, but innate unavoidable is much more of a problem.
Oh wait, I just remembered 3 more counters to unavoidable: daze, stun, and disable. Unfortunately extra daze conditions does not mean extra daze rolls, and disarm only works on equipment.
So we have helm of fear (warlock only), forcefield (forcemaster only), jinx (which refunds everything so they use the same attack again), fumble (which gets discarded after use and is too mana intensive to use that many times against the mage or brogan or similarly high-level creatures), intercept (only 4 creatures in the game that have it, all of them can only stop one unavoidable attack per round, all costing at least 11 mana).
With 3 fumbles (21 mana and 3 mage's quick actions against unavoidable mage attack), 3 jinxes (9 mana and 3 mage's quick actions), 1 intercepting creature (11 mana and one full mage's action), you've used 41 mana and about 8 quick actions to block (6 unavoidable attacks +1 unavoidable attack per round after intercept creature is summoned)
6 flameblasts costs 30 mana, 6 quick actions and about 12 spellbook points if you're not trained in fire.
3 jinxes and 3 fumbles costs 30 mana, 6 quick actions, and 12 spellbook points if you're not trained in arcane or mind.
However, many flameblast users will be trained in fire, so 30 mana, 6 quick actions and 6 spellbook points.
If you are trained in arcane but not mind or mind and not arcane, you pay 30 mana, 6 quick actions, and 9 spellbook points.
So being trained in the school of an unavoidable-counter is helpful, but it's not necessarily enough. Not unless you are trained in both mind and arcane, or dark and arcane/mind (since helm of fear)
So even though they seem to cost about the same, having to pay more spellbook points for the same amount of resources means you will have less of them than you're opponent would per spellbook point. I suppose the question then becomes a matter of whether conditions like daze, stun, and disable even out this inequality.
I'm guessing that they don't in the current meta, and your statement that defenses are better for strategies that want to take a bit of time to setup before going on the offensive as opposed to the more defensive strategies (pun intended) seems to support this.
Also, you're right that ballista can be taken down with acid ball and melee attack(s). Ballista's attack range is 1-2. The build I made that has an earthquake in it might like stomping all over some poor mage's ballistas.