Are wheels really a nightmare on an uneven surface/gravel? I intend to use it in the city but roads aren't perfect at all times.
Also, is this stroller uncomfortable for the child, since both 0+ and 6+ part look a bit thin?
To me, the short answer is yes and yes. Although the stroller is sturdier than many others in the category, the small wheels are not at all nice on gravel nor on cobblestones. On smooth surfaces you'll be fine, but on worse ones I see it rattle all the time (especially in old town).
For the child, it is uncomfortable first because it doesn't lie flat, and in the newborn mode it lasts about 4 months at the maximum (then the non-sitting baby is in the half-sitting position, damaging its neck and spine and/or breathing worse), and the older toddler is half-sitting in an awkward position and pulling itself forward because it is not comfortable, being "hung" on the harness.
At least, that is my personal experience and the reason I don't think this it worth the money for me.