I saw this in someone else's house rules and it looked like a good way to speed combat and make it more cinematic, but I'm worried that it would do something bad. What is the problem with it.

Critical Wounds : Targets suffer critical injuries when they take damage in excess of their wounds or that exceeds, from a single hit, double the Toughness bonus . On a critical hit, reference the critical table (adding existing critical wounds)

I just read the threads on modifications people made to how TB worked in soaking wounds. The implication is that it's VERY easy to break the entire system by changing anything, so I probably shouldn't do the above.

The implication is that it's VERY easy to break the entire system by changing anything

How? The only possible result is that you'd have people dying faster or slower - and isn't this the purpose of the change? The injury mechanics are completely independent from the rest of the game, so I don't see how it could have unwanted side-effects.

I just read the threads on modifications people made to how TB worked in soaking wounds. The implication is that it's VERY easy to break the entire system by changing anything, so I probably shouldn't do the above.

What? Again , I created a thread for the expressed purpose of discussing potential cascade issues with TB-Soak. In the course of, what, 8ish, 9ish pages, there wasn't a single example of how this might break the system.

Many of the various mechanics in the overall WH40kRP system appears to be relatively self-contained and it's actually very robust.

It makes high TB even more nice to have because of the doubling. TB 3 = 6 (Very bad). TB 5 = 10 (Extremely good, relatively speaking). With higher TB (let along UT) it gets a bit silly, but then again UT is already quite silly