As a person who has had to try and make several 'lumpy' runways and airfields flat (because that is all FSX can take), I can confirm that FSX is no different to any other MSFS.
If you see a 'humpy' runway (like the recent Fly Tampa Grenadines), they are using the background texture tiles and it will not (without great trickery, possibly

) support MS based AI or allow to you select it as a runway from the main FSX menu.
I have had hours of 'fun' trying to make Tangmere flat enough, in real life the whole station is on a gentle slope that drops about 20 metres across the airfield. I still cannot get it to look correct!