YES

TRS:

  flatten(nil()) -> nil()
  flatten(unit(x)) -> flatten(x)
  flatten(++(x,y)) -> ++(flatten(x),flatten(y))
  flatten(++(unit(x),y)) -> ++(flatten(x),flatten(y))
  flatten(flatten(x)) -> flatten(x)
  rev(nil()) -> nil()
  rev(unit(x)) -> unit(x)
  rev(++(x,y)) -> ++(rev(y),rev(x))
  rev(rev(x)) -> x
  ++(x,nil()) -> x
  ++(nil(),y) -> y
  ++(++(x,y),z) -> ++(x,++(y,z))

max/plus interpretations on N:

  flatten_A(x1) = max{7, 2 + x1}
  flatten#_A(x1) = max{0, 1 + x1}
  nil_A = 8
  nil#_A = 6
  unit_A(x1) = max{1, 1 + x1}
  unit#_A(x1) = max{6, 7}
  ++_A(x1,x2) = max{4, x1, x2}
  ++#_A(x1,x2) = max{3, -1, 3}
  rev_A(x1) = max{6, 1 + x1}
  rev#_A(x1) = max{4, 5}

precedence:

  flatten > unit = ++ > rev > nil