My first 1800 (a '72 E) had the cursed BW35 slush box. (hey - the car was cheap!) Despite it working fine when I got it it began to slur shifts and slip a bit in 1st gear. I honestly didn't abuse it, just did plenty of full throttle accelerations. It quickly went downhill once the slipping started so I sourced a junkyard M41 and swapped it. The car was much quicker, got better mileage, and worked like a charm for several years.

Until someone ran a stop sign and bent it into a 'U' shape....

I had the same thing (tranny go bad, no the stop sign thing...) with a '73 145E I once had. About 4 months of use then slipping and slurring. I put a regular 4 speed in that car. Enjoyed it much more after that. My parents put 280,000 miles on their '70 145S, however, and never even had the transmission adjusted as far as I know. I guess the easier use and less HP of the B20B helped?