Any serious torque increase will reduce reliability of the entire drive train, if used much, and there is no point arguing that one, BUT the issues are 1) by how much in the real world, and 2) do you care, over the life of the car?
I suspect for a road car (which, unlike a track car, you can't be maxxing all the time) there will be all sorts of other reliability issues, or rust, or accidents that will make the car have only scrap value before a modest torque increase has any effect.
That being said, if you go to serious torque, you could get into potential sudden, catastrophic failures for parts of the drivetrain that could not have been easily upgraded. But even then, if you are keen, everything can be rebuilt or replaced.
It also depends if you want to keep the car for a long enough time such that any resale value is irrelevant....
I'm never going to mod my humble MPi (not much scope anyway) but the beauty of all Ups! is that they are sufficiently inexpensive to pop into the recycle bin after say, say 5 years, so mod as you see fit!
On the other hand, if you were keen to mod, say, a new M3, you'd seriously have to look at the effect on resale value when you wanted to sell it, or the cost of fixing it when you break it, and whether you really wanted to keep it long enough such that depreciating it to zero value was no big deal... For me, £75k is way more than my ego is worth - I'd much rather have bought an Up GTi, and had fun uprating it, stage 3 or whatever, as the cost is relatively peanuts. And you really don't want a big thing like an M3 in West Cornwall, trust me!