Show me a single card in the current format or even last format that did well with an attack that required you to flip to do any damage. You won't be able to find one. Flipping coins to do damage isn't reliable and hasn't been seen since Exeguttor FRLG was able to flip a billion coins to offset the odds.
Blissey is different. It had a 100HP basic, the biggest basic that can evolve in the format. It has 130HP, the most HP on any stage 1 in the format. It has the cheapest attack you could ask for. And it can do unlimited damage.
Porygon Z is nothing like Blissey. It only has 110HP, 20 less than the stage 1 Blissey. It's a stage 2, which makes it harder right off the bat to survive in the current format. And you have to flip heads to do damage, unlike the guaranteed (and pumpable) damage of Blissey.
You get to flip 3 coins. The outcomes can be:
3 Tails: You do 0 damage. Thanks for playing.
2 Tails, 1 Heads: You did 40 damage for 2. Average...on a stage 1. Not even worth attacking mid-late game just to do 40 damage.
1 Tails, 2 Heads: You did 80 damage. You KO'd some stage 1's but most other stage 2's are still alive.
3 Heads: with a 12.5% chance of happening, you KO a lot but not many of the 130HP stage 2's. You got lucky enough to flip 3 consecutive heads and still don't KO everything in the format.
I know this doesn't account for it's Poke-Power, but you can't always assume it will be active. The same way you can't always assume you will have a Lake Boundary in play. The same way you can't always assume that you will have the right basic energy to discard to get the opponent's weakness.
Porygon Z lv.X has a nifty poke-power and a really nice one. But it brings no other attacks to the table. It's the only lv.X/non-lv.X combo that gives you no extra options for a higher cost. Because of that, you are forced to flip heads or go home.
And the worst part is that you can't increase the number of flips.
I heard someone say that they don't play the deck that could get lucky one day and do well. They play the deck that will do well consistently. Porgyon Z is not going to be able to do well consistently with it's ONE attack. I don't see many players using it for any kind of offensive attacker.
If anything, people adore the lv.X because it reminds them of a broken card in the past. But that was in the past, and the same broken cards in the past might not be as broken now.
Nothing is fact. All is opinion, but well-supported opinion.
edit:
Laugh all you want. I don't really care. If you like it so much, play it, and do well with it. Play the card you like and be quiet. But don't rub your words in people's faces about how good you THINK the card is and how stupid you feel people are by underestimating it. Be courteous and play it when it comes out. If it's as good as you feel it is, you will do well with it. If not...
:/