The older multi's in the A4 B6's were problematic but B8 A4's and C7's A6's have been fine with beefed up internals.
I had a new 59 plate 2.0tdi Multi which did have a fault which was sorted but it was not the reported issues just a random fault. I now have had my A6 Multi for a year and no probs, I really do like the box and would not hesitate to have another.
The latest 2.0TDI's now have the s tronic so the multi is now completely obsolete
MPG is always going to lower than book and the multi is nothing different. I do get between mid 40's and low 50's
Paul