Hi Mark,
As you are no doubt aware, range estimates are really difficult to make as every rider's effort level is different and terrain / weather can make big differences too.
That said, we're both in our 60's so hardly in the first flush of youth and no longer putting out anything like as much power as we used to in our younger days. The all up weight of the tandem plus riders is probably around 165Kg and the motor has no trouble propelling us. To try and give you some indication though, we rarely use more than power Level 2 of the 5 Levels available. Level 2 (assistance to about 11 mph) has taken us up any hill we have encountered - we live in Edinburgh and there are plenty of hills! Other than hill climbing / headwinds, I think most of the time, we're normally riding above the speed that the motor would cut in, so not using any battery power.
One of the biggest benefits of the motor is starting off from rest or regaining speed after having been baulked by dog-walkers on shared paths. The motor just whisks you back to cruising speed in no time at all and there's no stress on the old knees. The throttle control is great for quickly boosting speed to make sure you get safely through traffic lights or not obstructing cars when setting off from the lights. Hill starts are no longer an issue and as you'll know, that can be 'fun' on a tandem.
I think the furthest we've travelled without recharging was about 50 miles on our 12Ah battery. Fully charged the battery shows 54.2 volts on my multimeter and the lowest reading I've taken is 47.1 volts after almost 50 miles covered, so it still had a bit to go. For us, even with a strong headwind (no shortage of them in Scotland) and a fair bit of hill climbing, the battery still has plenty of power after 30 miles, albeit with a bit of voltage sag on the steeper hills.
We've done over 2,000 miles now with the motor and it's a great thing to have. You'll definitely go out more on your tandem if you do decide to go ahead.
There have been very few drawbacks to my conversion and I'd stick with a rear hub drive if I was doing it again. I have had a few broken spokes, but that isn't unusual on a tandem and as the hub is only a 36 hole, it was not entirely unexpected. The original (tandem specific) Shimano hub was a 40 spoke which is a fair bit stronger.
I recently rebuilt my hub motor with a Ryde Sputnik rim / Sapim strong spokes and I also used spoke washers at the hub to try and get a decent 'set' for the spoke heads. I had a look at the wheel building video on the 'Grin' website
https://ebikes.ca/learn/wheel-build.html and I laced the wheel as a single cross with all spoke elbows out, so I'll see how that goes. The wheel has done about 200 miles since the rebuild and all seems well.