I've been theorycrafting the applications of Breacher teams and they are a very difficult unit to justify. This is because they fall into the oh so familiar pitfall of "It's good, but I can get better units for the points".
Point Cost - This is the biggest concern with the unit. Your team has to be mobile, so giving a devilfish is almost a requirement. After upgrades, troop increases, etc, the squad itself comes close to 200pts. At that point you can start to argue "what else could I take instead?". You could find plenty of solutions to that question, most of which are often more durable and more shooty than the team itself.
Armament - The Breacher team has what is essentially a pulse rifle with an up-close and powerful 5in or less blast weapon. This is what makes them scary. Getting an enemy unit in range of that can melt MEQs. However, getting into range of the target itself is the biggest problem. They also lack ways to get close enough without help from another unit. More on that.
Survival - The edition of the guardian drone is what makes this unit durable. Having a 5++ save isn't amazing, but it's pretty damn good for normal rank and file infantry that are under 100pts. So while they can certainly take punishment, they are still on a rather weak statline. T3, 1W and 4+ saves still make them pretty weak for the job they are trying to do, which is getting as close to the enemy as possible.
They do have some benefits that could be applied though. As was mentioned, Darkstrider is a useful edition to this squad. It is possible to do some shenanigans with his outflank, and again with his special rules. It is important to remember though that the unit is still very immobile, which again, is the hardest part about this unit.
The Hunter Cadre / Hunter Contingent would probably be the best place to have this unit. As was mentioned the combined firepower special rule when used in conjunction with the devilfish, detatched gundrones, and breacher team, gives you BS4 (except for drones) for those three as well as ~24 S5 shots at the very least. However, this alone is not enough to make the unit do well IMO. You would have to make use of the Ambushes and Feints special rule from the Hunter Cadre so that they could get in range for their AP3 profile.
And there lies the problem. Without the AP3 profile you're better off taking a Strike Squad or something else. If you cannot reliably get in range to use that profile, the unit is almost a waste of points. Try it yourself in practice games or just with your own units on a table. Even with Ambushes and Feints, the Breacher team has a hard time getting into a solid shooting position to use that AP3 profile.
Sky IS Falling, T'au WILL Suck, Sell Me Your Models