I also have some concerns about the rear mounted spare wheel but I can't give you a final thought at this stage.
I only did 200 km from the place I bought it to where it's parked now. Everything was normal to me. Sometimes I forgot I was towing a caravan.
The wheel weighs 13,5 kg and when and I took it off I only noticed a small change in the nose weight. The caravan is still empty. I have the intention to place two bikes in the front and this may compensate the spare wheel.
Originally the wheel was mounted bellow the caravan, but the previous owner changed it to the rear in order to have a better access.
From 2005 to 2008 I guess some Eriba models came with a rear spare wheel. I don't know why they stopped doing it.
In conclusion, first I have to know better this new toy, have it full equipped for traveling, and after some kms on the road I will give you a better answer.

That's interesting - are there other countries that don't allow rear mounted bikes? Are you allowed a rear carrier if no bikes on it? I know you have to have a red/white warning board at the back if carrying bikes in Italy as did that on a previous Triton.

I'm about to add a rear carrier - I already have a front bike mounting and intend to balance the load with one bike forwards and one at the rear. With the intention of swapping to electric bikes this seems sensible due to the extra weight... And there's no way I'm lifting them onto a roof rack