Bonn, like the i3 and other mid/late-2000s UK sewer infrastructure installations, failed to even meet the baseline metrics set by first generation micro trenching fibre rollouts - which, as we know from Google Fiber in Louisville, KY, is an extremely low bar to hit. Micro trenching was a decidedly inferior solution to instead utilizing existing above-ground conduit or doing proper in-ground installations with full fat trenching or similar methods. Google’s micro trenches failed so badly in Louisville that they fled the town and paid $3m+ in remediation fees; this is the tech that is SUPERIOR to sewer installs, according to the results from Bonn and other Western European municipalities.

TL;DR: AFAIK, there is not a single trial run that has found sewer to be the best option for any of the following metrics: initial installation cost, ease of installation, lifetime cost/TCO, maintenance frequency, and network downtime/reliability rates.