just had to replace the headset bearings on my madone slr (a lot earlier than usual) mostly from sweat getting into them from so much riding indoors during covid. considering that my MTB's generally go through headset bearings far more quickly than my road bikes, i'd either want the absolute most bomb proof bearings i could find or never run internal cables through the bars and headtube on my MTB. it's just a huge pain for an otherwise basic maintenance task.
i'm running etap on the madone so it was only brake housing i was having to mess with but just a ridiculously time consuming process for something that should have otherwise been a 20 minute job.
with that said, i know why you want to do it!