If there is smoke they are burning extremely inefficiently. The ones I’ve seen in operation emit zero smoke.
Mobile crematoriums? Reportedly used in The Donbas.
At least some crematoria routinely emit pollution. When my brother died in the summer, in the Inland Empire of CA in the early 2000s, I was told that cremation caused a lot of emissions. It was very regulated because that area is often smoggy. We actually had to pay a surcharge for his cremation because he was a big guy and would pollute more.