This is a solid take on a zombie plague for a sci-fi/supers setting. The premise is summed up nicely in the description, with an alien vessel from one of the Psi-Watch setting's nastier species crashing to Earth and letting out an extremely dangerous bioweapon (or an unfortunate combination of several different bioweapons - no one is sure exactly what the original cargo was, so it's quite possible the results of the wreck aren't what anyone intended). The disease from the wreck comes in two flavors - a weaker, airborne strain (dangerous to normal humans, but superhumans are extremely resistant and can shrug off the effects with minimal effort), and the far more potent direct injury/contact variant (which is the one that can infect superbeings and potentially make special infected out of them). In true horror movie fashion, things go pear-shaped immediately as everyone underestimates how fast the disease can spread, and the results are nothing short of apocalyptic.
The basic zombie from the disease is Resident Evil/Left 4 Dead type bullet fodder - animalistic, not terribly fast, and extremely vulnerable to damage. How vulnerable? Well, this was made back in the 3.x/d20 modern era, and the zeds still have an explicit critical hit vulnerability that stacks on extra damage. The special infected are a different story. They retain various degrees of intelligence, along with most of their original class/race/superhuman abilities, and stack extra capabilities on top of it. These vary from the traditional Tank/Brute type of heavy hitter up to the Lancer (an undead superhuman with a built-in ranged attack of energized pus). They're decently tough as is, but do require some tweaking.
The downsides to this supplement are as much due to editing and rules consistency as anything. Concept wise, things work great, but there are references to Alpha zombies (implied to be a tougher form of the regular zombie but not quite on the level of the special infected) which aren't statted up later. The race that made the bioweapon in the first place is implied to be immune to it, but this is only referenced offhand and is never explicitly stated. The special infected have a couple of wonky bits in their stat blocks that don't really line up with other information in the supplement.
This is still a great supplement for the price involved. You'll have plenty of stuff to work with here in terms of ideas and mechanics, but some assembly will be required to make it fit for your particular game world. Tweaking it to Pathfinder 1st or Starfinder won't be too much of a problem, and if you still play 3.x/d20 modern it will work as-is.
The author has indicated he plans to revisit and update this supplement eventually. If he does, other than cleaning up the rules language and consistency, I'd recommend expanding the types of special infected as well as adding in charts for additional random mutations that can affect both special and regular infected (that way, it covers the ground for the other types of special zombies seen in various survival horror games, with tentacle or blade arms, etc.).