Tear out is always a problem where the cutting edge exits the wood. Since a spinning router bit cutting edge exits both sides of the work piece, you can have tear out on both sides of the work piece. A sacrificial strip attached to both sides of the work piece can keep this from happening, because it will prevent the wood fibers along the work piece from breaking free as the cutting blade exits the work. The sacrificial strip will suffer the tear out instead of the work piece.
It is possible to significantly reduce tear out when router cutting dovetails by climb cutting the surface of the work piece before plunging in to cut the rest of the dovetail. In the climb cutting process, the blade of the bit is cutting into the work piece and removing the wood without pulling outward on the surface wood fibers. Where the blade exits, the wood fibers have already been cut, so no tear out occurs. When you go back and make a second pass to deeply cut the dovetails, there is solid wood adjacent to the cutter exit point to keep the adjacent wood fibers from tearing out.
Since plywood is many layers of wood with each layer having it's grain oriented 90 degrees to the layer below it, you are performing both a ripping and cross cutting function at the same time. This puts additional pressure on the work piece surface wood fibers as the blade is exiting, making it more difficult to keep the surface fibers from breaking adjacent to the blade exit point. The plywood layers are also very thin, making the wood fibers that are not parallel with the layer surface much more prone to breaking. Regular plywood has voids and grain running in many directions in it's layers. There are areas completely void of glue between these layers, so it's difficult to avoid tear out when cutting it no matter what method is used to minimize it. Baltic Birch plywood has surface layers that are only about 1 mm thick, so will break very easily when not supported with a sacrificial strip, but using a sacrificial strip and very sharp cutters can produce success at slower cutting rates.
Avoiding the "cutting in both directions" problem to eliminate the need for the two sacrificial strips, makes using a table saw the better method of cutting box joints, since the cutting edges of the blade are always going in the same direction through the work piece. Only the exit point of the blade needs the wood fibers of the work piece to be held from breaking as the blade exits the work piece, so a sacrificial strip is only required on the blade exit side of the work piece and the cutting edges of the blade are always cutting in the same position on the sacrificial strip, so the strip can be used multiple times if held in this position, without a problem.
For cutting box joints, I've found that I can get significantly less tear out and better cuts
using my Unisaw together with an Incra I-Box jig and a saw blade or blade set that has the FTG Flat Tooth Grind profile than I can when using a router and any kind of straight bit with an up-down spiral, or otherwise, cutting edge. I can even cut Baltic Birch with very little tear out using this table saw method. For box joints larger than 3/8" I have used my Freud Dial-A-Width dado stack, which doesn't produce the perfect flat bottomed cuts, but it is less noticeable when the box joints are this large.