Given this...

dataset = Dataset[{
   <|"a" -> 1|>,
   <|"a" -> 2|>,
   <|"a" -> 3|>,
   <|"a" -> 4|>,
   <|"a" -> 5|>,
   <|"a" -> 6|>,
   <|"a" -> 7|>,
   <|"a" -> 8|>,
   <|"a" -> 9|>
   }]

And given that this works...

dataset[[1 ;; -1]]

Then shouldn't this work...

dataset[[1 ;; -1 ;; 2]]

For me, the last input returns...

Missing["PartInvalid", 1 ;; -1 ;; 2]

Just by the way, all three of the above statements work if dataset is defined as follows:

dataset = {
  <|"a" -> 1|>,
  <|"a" -> 2|>,
  <|"a" -> 3|>,
  <|"a" -> 4|>,
  <|"a" -> 5|>,
  <|"a" -> 6|>,
  <|"a" -> 7|>,
  <|"a" -> 8|>,
  <|"a" -> 9|>
  }