You have to look at 2 things: 1. support
i.e the strength of the subsurface to hold whatever is on top of it (and here, 3/4" plywood is sufficient to hold most things including a person standing on it) and 2. a Waterproofing
Now cement backerboard (CBU) is used under bathroom tiled floors and walls to provide both support and waterproofing (to some degree). CBU isn't waterproof in itself but it is water-resistant
that means it will repel and not hold onto water too long. A waterproofing membrane is normally placed on top of the CBU to waterproof these area.
So, let's say we've already established that support is not really needed because the plywood can hold its own. But is such a system waterproof? No
. So on a kitchen countertop, we need 3/4" plywood - and a waterproofing membrane. This is available, and one example is called "Ditra".
There are edge mouldings available that are just the right height to cover 3/4" plywood+thinset+Ditra+thinset+tile. 90-degree mouldings are also available where the tiled flat surface meets a backsplash, or the wall - either way more tile.
With your plywood+CBU+thinset+tile, the overall height might mean that you'll have to custom-make an edging that goes with it. We get back to my original statement in that no CBU is really needed on a kitchen countertop. It is not a strengthening element...
An important point is the sealing
of the grout between the tiles; sealing here will prevent stains etc from penetrating the grout. The tiles themselves may or may not need sealing, depends. But to offset that, the word 'tiles' can of course include granite, slate, porcelain - any
tile you want. In my picture, we used porcelain, but elsewhere we spent the $5 each and bought granite tiles. Do the math vs a granite slab at something like $50-80/sqft....plus installation charges of course.
Tile countertops aren't quite a DIY project but on the other hand, once the planning and prep work is done, a good contractor will give you a new countertop in a day.
Hope this helps...