The early-WW2 infantry anti-tank weapons, such as anti-tank grenades and anti-tank rifles, were quite ineffective, particularly those used by the Germans.
But the late-WW2 infantry anti-tank weapons, such as the Bazooka, Panzerfaust and Panzerschreck, seem so much better:
But how effective were they really?
The Bazooka had actually quite a low armour penetration value, around 70-100mm, but it was the first of its kind (that was mass produced and employed anyway) and also reusable. The Panzerschreck on the other hand had around 200mm, which is crazy high considering the most common anti-tank gun/tank cannon for the Germans around that time was the 75mm and had less penetration (though of course also able to fire at much greater distances). The Panzerfaust was not reusable, but also had extremely good penetration.
Also, what's up with the Germans producing and using both the Panzerschreck and the Panzerfaust at the same time? That just complicates the supply chain. Why not go with just one? I understand that they (probably) developed both to explore two different approaches to this kind of weapon and see which one is better, but once they figured out which one is better, it should be better to just stick with one, no? So then the logical conclusion must be that they were either stupid, or that those two weapons filled two very different roles in German infantry anti-tank doctrine. So, which one is it? If its the former, then which one was better? The Panzerschreck or the Panzerfaust? And if its the latter, then how were they used differently doctrine-wise?
Speaking of reusable (Bazooka, Panzerschreck) vs. single-use (Panzerfaust) infantry anti-tank weapons: The main difference here seems to be fixed (short term) costs vs. variable (long term) costs. The reusable weapons would have a greater fixed cost, as the launcher is more difficult to manufacture, but the variable costs would be lower, as one could just have to keep supplying the already existing launchers with new ammunition. You just need to produce new ammunition, as well as replace any worn out or lost launchers, but you don't need to always replace the entire weapon. Meanwhile with the single use weapon you would need to always produce an entirely new launcher as well as ammunition to replenish the troops. Said launcher would overall be cheaper to produce than the reusable launcher, as the single-use launcher is simpler in its design, leading to lower fixed costs, but once you have to use the weapon more than once, the reusable launcher should be cheaper. The question then really comes down to how much of the costs are due to the launcher and how much due to the ammunition. If the ammunition makes up 90% of the cost anyway, then whether you are using a reusable or a single-use launcher won't make that much of a difference.
Lastly, why were the Russians, or any of the other major participants of the war that faced a lot of tanks for that matter (Japan, Italy, Britain) so behind in developing a similar kind of weapon? Was it simply not needed that much due to various factors? Did they lack the know-how and their allies were unwilling to share their technical expertise with them? But then the question is why they couldn't come up with their own design, while the Germans were able to quickly copy the Bazooka (as the Panzerschreck) and develop their own kind of weapon (the Panzerfaust).
EDIT: Forgot that the PIAT also existed!
The idea of multiple types of man-portable anti-tank weapons being fielded side by side is hardly unique to WW2. In the Cold War for the US Army one might see an M72 LAW disposable rocket side by side with a 90mm or 106mm recoilless rifle. Today the AT-4 is the disposable system and either Carl Gustaf or Javelin for the reusable one.
All man-portable weapons are going to be a balance. Panzershreck was very good. It hit massively hard, was accurate and had reasonable range. It was also large, cumbersome and expensive, comparatively speaking. Generally speaking, it would be assigned to a specific anti-tank team who were given special training on the weapon and who could make the most of its capabilities, normally at higher than squad lovel. Panzerfaust, however, was a cheap and cheerful thing, lobbed by a black powder charge instead of by a rocket, which anyone could carry in addition to their primary weapon, but which was really only accurate at shorter range.
Thus they were complimentary.
Japan did create its own version of Bazooka after it started encountering the things in 1944, the Type 4 rocket launcher of which about 3,500 were built. However, as with a number of the other final generation of weapons such as the Chi-Nu tank, they were retained on the home islands to defend against the final invasion. You did indeed catch yourself on the British PIAT, which unlike the other self-propelled systems was a mortar.
The Soviets were their own special case, with 14.5mm anti-tank rifles coming out of their ears. There weren't intended to 'kill' a tank as much as disable it. They were surprisingly effective, mainly because there were so many of them. However, the RKG and RPG series of hand grenades did use a shaped-charge principle, as long as you could get within lobbing range. It would not be until after the war that the better known series of RPG launchers started proliferating.