Reasons for changing - Like all brewing systems the BM had some limitations/quirks/idiosyncrasies. Some of which could be worked around, some which perhaps linger as mild annoyances.
For me I guess there were a couple of reasonably major points...
1...the 6-6.5kg practical limitation to the grain in the malt pipe...mash efficiency notably drops off once you get above 6kg of grain in the malt pipe. Yes if you want to brew a bigger beer you can brew short, or do an iterated mash, boil longer....yes you can just about squeeze 7kg in if you use a "yield increase disc/filter" or you can add fermentables (D/LME) to the boil...but at the end of the day they are all what I would call compromises.
2...without wanting to drastically increase my production levels (at the time, I brewed maybe 17-18 20L batches a year) there were times that I wanted to brew a longer brew length...competitions, sharing etc etc. Yes, there was nothing really to stop me brewing say 25L on the BM...but that requires an increase in grain to achieve similar strengths and starts putting us back into point 1 above territory. Generally I now find I'm brewing 23-25L batches since moving to the G40....my limitation now is my FV size and I don't really want anything bigger as I wouldn't be able to haul it easily into my fermentation fridge.
Minor irritations...
3...extracting as much of the wort from the BM into the FV as possible...the 5L deadspace under the tap is a lot of beer to "lose"....yes I know grain is (or was!!) cheap and I could increase grain to get my required brewlength/strength but once more we start getting into 1 and 2 above....and you know...it still "hurts" to leave 5L of wort that could potentially be the best beer you've ever brewed in the bottom of the BM
I've tried other solutions such as fitting a racking arm to the inside of the BM outlet...reasonably successful, I've used the BAC Brewing springer system....great concept, works perfectly with plain water but the moment you start introducing sticky sugary wort and maybe some hops into the equation I found it didn't work so well...unless you put a pump on the outside tap and sucked the wort out...then it is perfect!! There are other solutions...Speidels device that uses the BM's own pump to drive the wort up a tube via the same hole that the pump uses to pump through the mash....this actually works very well....but you know what....its yet another device and piece of tubing to clean and sanitise!!
4...relatively low powered heating element...slow temperature ramp up times makes for a long brew day....I brew outside in my garage at all times of the year and despite having the BM ThermoCollar, and an additional layer of foil backed bubble wrap thermal insulation, the painstakingly slow 1 degree per minute heating rate does make for an extended brew day...another 500w on the BM20 heating element would be ideal in my opinion!!....the 2.9kw element of the G40 has proved to be a big improvement.
5...weak boil...again brewing outside in the winter you really notice this...effectively the same underlying reason as 4...the domed lid does help in this regard. I have no such issues with the G40 boil.
6...cleaning....getting behind those spiralled BM heating elements was a real pain...especially with leaf hops...cleaning the non visible side of the spiral elements also wasn't easy. Cleaning the G40 is a breeze by comparison.
Don't get me wrong....I'd had my BM for 6 years and done roughly 100 brews on it before deciding to chnge....it was and still is an excellent piece of kit but, times move on, other newer devices offer alternative ways of doing things (and, no doubt, also come with their own set of compromises!!!) and potentially improve upon older models.
I have no regrets about making the change.