it definitely was meant to be stronger, and it was, holding up 450hp and close to 600NM is far more than the 360 and 420NM of an S8. Problem was tuning which would put even more stress on the gearbox by the increased torque and early ones had a leaking seal on the torque converter which eventually, if undiscovered quickly, would lead to gearbox failure and all that coupled with the rubbish lifetime oil meant planing for disaster
.
if you want to get one, look for one which had gearbox and DRC replaced, and you should be ok, those are the big tickets with the RS6 C5, otherwise is just like any other V8 from Audi, apart from brakes, which are quite expensive to replace. Look on rs246 forum and you'll find whatever info you need, but be warned that that power is addictive and comes at a price, once you accept that you're good to go and enjoy it.