You could also have commented that two of the cards in my combo have a random effect, making it even less likely that they do what you want. The point is not "this will happen every game", just that it is possible.
As far as sideboard tech, I did mention in my post that those decks get hosed by certain cards.
Dredge loses out to any form of grave hate.
Affinity's whole deck gets turned off by Stony Silence, including some of their mana.
Yet those decks remained popular for years and required bannings to keep them in check. Why?
Because they are so proactive. They only have to worry about executing their own strategy, the opponent is the one who has to worry about thinking on the fly and finding a way to stop it.
If the opponent doesn't find their tech fast enough, they lose.
If the opponent draws too much sideboard tech, they probably still lose, because those cards are blanks beyond the first. The proactive deck will just play and attack with creatures while the opponent draws cards that might as well be basic lands.
Beyond the decks I already mentioned, look at decks like Ad Nauseum and Storm. They fold to sideboard tech even harder, require a specific combination of cards and also require at least a little luck. But they remained popular choices over many years. Why?
Other good deck comparisons for the ability as written would be Dredge (gets power on tbe board for free) and Affinity (Hard to get rid of their constantly recycling power boosts).
What these decks and Death's Shadow have in common is that they are "make them have it" decks. The opponent has a very narrow window to respond, often only one or two turns, and they have to have the correct type of interaction. If they don't, they lose.
Yes, these decka are completely built around tbeir core strategy and yes, they fold hard to certain sideboard cards. But you can't say that they aren't effective. They were borderline broken at their height, because they circumvent the normal cadence of play.
As an example of how bad it could be, imagine this scenario:
It is the finals of a Modern tournament. You are on the draw.
If each Reincarnation card adds an average of 3 power, you are dead before your first turn. The higher the power added, the fewer Reincarnators you need to get into the graveyard to make this possible. Since there is no cost, it doesn't even matter which Reincarnators get pitched. They might have discarded and utilized cards of all 5 colors, even though their deck can only produce red mana.
This is Christmasland, of course, but there is no "dies to removal" in this example. There is no response at all. Even a nominal cost would prevent this from being possible.
Th originally proposed ability is pretty much Haunt, but spelled out a little differently.
Tormented Hulk1GG Creature — Human Beast
Haunt (Whenever this creature dies, exile it haunting target creature.)
The creature haunted by ~ gets +3/+3.
Edit: I thought the intention was to have it shrink itself on the field, but give the full +8/+8 when it reincarnates. But I see now that, as written, it gives the reincarnated creature both +8/+8 and -5/-5?