I have the uppababy vista 2015 and I love it. But it does have limitations.
The main sest has to go up top and the rumble/second in front/below. You can't put the main seat in the front/below.
You can put the car seat on either position provided you buy the adapters for the car seat AND the upper/lower ones too.
The board is great and sturdy. The telescopic handle means it's easy to push and they can stand up with good headroom.
I didn't buy the second seat in the end. For the first 5 months I used a combination of the carry cot or car seat on the front/bottom with the toddler in the main seat facing me.
My toddler is 16kg and my baby now 5 months and 8kg. I was concerned that the 16kg would be too heavy over the front wheels to go up and down curbs so have just persevered with the car seat and main seat or the main seat and buggy board.
My toddler is now 27 months and is good on the board.
If we go or for the day, I bought a side by side double with the money that over saved in not buying the rumble seat. That way they can both sleep for their 2 hour lunch time nap without concern.
The basket is huge and a godsend with two.
It's a bit wider than the previous model to compensate for the height when used as a double, but isn't a problem
I've had three on, carrycot/main seat/ buggy board. Handled really well.
I bought it when pregnant with my second as I just couldn't carry my toddler anymore, and it was a great single too.
I really recommend it as a versatile Pram, there is no perfect, but this really is great in my opinion. Good Luck!