We used MB urban from 3wks - sling occasionally, but ds quite heavy, and not good for long dog walks.
MB will happily go through foot high water/mud (OK, it sometimes feels as though you're ploughing, but it comes out fine the other side, just good exercise). It goes over everything. It will go through foot high rye grass.
No brakes to get stuck up with mud/grass - wrist strap for steep hills.
Ds in normal buggy from 3wks - no carrycot fitting, and never used car seat add on either (couldn't see the point - not meant to leave baby in seat for too long so time out walking restricted). Just need good winter clothes and or cosy toes for cooler walks.
Only problems we had over 2.5yrs use - 2 punctures and once lost the bearing that held the back wheel spindle (packed flat for holiday packing).
We got the urban so that we didn't have to push the handle down to change direction - but still could put front wheel fixed for very rough terrain (rarely used this evening though we went properly off-road quite a bit). Turns on a 6p and could push (and steer) with one finger.
Ds found it too smooth in MB if we went on the pavement and had to go up and down the verges to make it OK for him to go to sleep
We never got a stroller so MB was used full-time and until ds decided that a bicycle was a preferable form of transport.
We're keeping it, just in case we ever manage to have a 2nd dc!