I think the best way to look at this is, which would you buy, a new home built on a site that wasn't used for landfill or a new home built on a non-landfill site?
Inert means it doesn't break down and give off methane gas.
It was probably landfill from construction sub soil, or inert construction waste.
Nevertheless you will not know how or how well it has been compacted and whatever the foundations (they must be a reinforced raft or piled) there could still be subsidence.
Asbestos is inert, but it is hazardous!
Your home insurance might also be higher too.
If you now, even before buying "don't trust the developer for a straight answer" you should be buying from them at all.
My advice is don't buy!