After rummaging posts and threads about Zac, I find that many complain about his passive(The revival part) and say that it's useless, weak, garbage, trash and whatsoever. But i think that his passive is great, even with the 5 minute cooldown(I wish it were just 4 or 3.5), its still great. Maybe people just forget that Zac was designed to be a tank and not meant to carry games. He is supposed to stack health and resistance in order to strengthen his passive, not build a lot of AP or Magic pen and then complains that his passive takes long enough to revive him, or that his bloblets die easily especially from AOE abilities.
I think the main problem comes with Zac's role, i find people, specially in my country, try to use supports, tanks and bruisers as carries, I'm not saying that it's not viable or useful but Zac's role will affect his passive greatly. The thing that differentiate tank Zac from AP carry Zac is target focus, in a team fight, who would you rather kill first, the tank or the AP carry? Of course the one who deals the greater damage, but seeing Zac's kit, I think he doesn't even need to build AP, his W is his main source of damage and it scales with his enemies health.
This is one of the biggest complains that even I thought that it was absurd, Anivia had an egg with her full health, a time limit of six seconds, and gets revived with the health remaining. There is a lot of difference but as people tend to forget, Zac is a tank, his passive is not meant to revive you on all occasions, it is meant as a back-up just like Volibear's passive, its a mechanic that helps you in case you got focused first in a team fight or got too early in initiating. Of course Anivia would have 6 seconds, she's a carry and killing her is a top priority, while Zac on the other hand is a tank, who the hell would waste time on a tank, and who the hell would focus the tank to die first? 8 seconds for Zac is balanced in my opinion.
12% is small and 50% of resistances is very small I agree but if you analyze it further there are four bloblets and that is a total of 48% of Zac's health even with only half of his resistance, it would take at least 3 hits to kill those things and I believe that Riot said that his bloblets takes only 2/3 of the damage from AOE abilities. You might say there are 5 enemies, and doing at most 12 auto-attacks or 3 AOE abilities is very easy to do but you have still four allies and as a team that focused the enemies tank to death will have their abilities on cooldown and would be left defenseless against the remaining enemies and would you rather waste your attacks or abilities on a poor and struggling to survive Zac or the remaining ones who are still ready to fight?.
Yes, Zac's bloblets are very vulnerable to AOE abilties and even with the 1/3 damage reduction, against an AP carry, those blobs woulds sizzle and burn in just one go and its very hard to miss knowing that they all meet together. But then again just as I said, would you rather hit 4 poor,cute,defenseless, helpless and harmless Zac blobs or a 4 enemy Champions? Also another counter to this is that in a team fight your blobs can get hit by AOE abilities if your allies are close, that part is a job that you leave and trust to your teammates, your team should know to stay away from you while reviving.
The health gain of Zac seems pretty doubtful going as low as 10% and hitting the 50% mark is kind of hard that I wish that Spirit Visage' passive would increase the health restored from 10%-50% to 12%-60%. And even if you do get revived, 8 seconds have passed and 8 seconds in an RTS game is long, real long and a lot of things could have happened, the enemy disengaged, your team won or your whole team got killed and the enemies are waiting just for you to stand again. And then again, whatever will happen in those precious 8 seconds are in the hands of your teammates. In the scenario that you're remaining teammates are not outnumbered, you're still gonna be very vulnerable with a minimum of 10% health, but with two cooldown abilities of 2.4 and 4.8 seconds with full cooldown reductions and a passive that heals, gaining health back would be no problem as long as you hit the enemy and they don't hit you, just don't forget to grab the blobs.
I believe that Riot did a good job in creating a champion that involves a high risk, high reward gameplay and I find Zac's kit to be focused around this gameplay. Yeah you lose health by using abilities but grab the blobs back and you gain more than the cost. They may step on your blobs and prevent you from healing but that ruins their positioning and could have used that time window to attack or use abilities. They might hit you while trying to grab your precious but hey, you're a tank, they're just wasting those attacks and abilities instead on using it on your teammates, but at least you did your job as a distraction. Zac not only becomes as a disruptive tank with his cc's which only last for a second or less but also with your passive, making you're enemy choose whether to step on your blobs to prevent you from sustaining and ignoring you cause you are not a priority as a tank. Whether to destroy you're bloblets and get a kill or use their offense with efficiency against your allies. Should they commit on killing you after full focusing thinking that you were alone or aim unto your incoming team. Zac's passive in my opinion is really great and tricky and I am really excited to get that piece of slime on his release. He might not be able to carry or get pentakills, but that's fine, he's a tank and he is meant to help his allies get the kill.