Had the distinct displeasure of flying the BN2 MKIII around Vanimo/Madang back in the late eighties.
Yep, it was really a truck and heavy on the controls but could carry 17 pax from a decent sized strip.
On the coast it was fine, but trying to levitate it from a strip such as Lumi or Anguganak required deft of hand and alot of faith in one's weight estimation and loading!!
Was great to have an extra 15 knots or so TAS, but on the downside doing the balancing act walking along the spine of the aircraft (on the roof) to check the rear engine oil level was quite demanding at times.
With regard to a Trislander that went for a splash, I remember seeing the rear engine exposed at low tide somewhere in the Madang Province.
From memory a few drowned simply because they couldnt take off their seatbelts.
Apparently Norman (of Britten-Norman fame) had on the drawing board what was to be named the "Mainlander" - basically an oversized Islander but with 4 engines and could seat 24 pax.
The mind boggles!!!