For active landfills, yeah.

For decommissioned and capped-off landfills, nah, not if they've done it right. (And given that this is a regulated industry, it's very likely that they've done it right.)

In my experience in the American Southeast, Academy of Model Aeronautics airfields are often built on top of decommissioned landfills [0]... so I have spent a lot of time standing on and around these things. I've never noticed any odor, soft spots in the terrain [1] that is the cap, or any of most of the other weird things one might think of when one thinks "site of a former landfill".

[0] Why? Because the land is quite cheap, and a ~100 foot strip of tarmac, a few little structures (sun shelters and the like) and parking for a couple dozen cars isn't all that much weight.

[1] To be fair, the runway did have to be worked on every three, five years as the terrain under it settled. But bear in mind that we're talking about bumps in a runway for model aircraft, so the amount of settling was actually not very large.