For the first time since buying my Bernina 820, tonight I wished I still had my Pfaff QE 4.0. Why? It has one stitch which is perfect for the applique I'm doing, and I can't replicate it on the 820! It's a zigzag that has an inbuilt change of stitch width (with a fairly long repeat and random effect). I even double-checked my older Pfaff, just in case it had that stitch, but as expected, the answer was 'no'.
I've tried several methods, including programming a series of zigzags of varying width, but when doing that, the 820 insists on an extra stitch after each zigzag, which ruins the effect. My best results (shown) so far have been by using the regular zigzag stitch, reducing the pressure of the presser-foot, and rapidly zig-zagging the fabric as it feeds through the machine - but unsurprisingly, this is a bit hit-and-miss when it comes to actually securing the edges of the applique. So in this regard, it's lucky that I'm going over each edge twice, blending threads - though it's not fun to do. The photo below is blown up large to show the stitch details - the piece of applique fabric is only about 1/2in wide at the top, tapering down to about 3/8in at the bottom.
However, I wouldn't swap my Bernina 820 for anything, and it's somewhat excessive to want what is still an expensive machine for a stitch I've used on only one other quilt out of almost 200 - and in small amounts on both!