I have always loved Tau for one overriding reason, they are a skill army. Even in 4th Tau was never that easy to get right and in 5th it is even harder but that is what attracted me to them. With other armies you have the ability to vary army builds so they exploit all three phases of the game, with Tau you basically have only two to work with (movement and shooting). If you do not deploy and then play tactically pretty much perfectly you will lose or at best draw. With Tau you also need to know your opponent armies very well, their movement and deployment capabilities, their special rules ( we have no psychic defences for instance and poor LDS in many units), weapons ranges etc.
With armies like Marines you can make quite severe mistakes and still perform well, simply because even their base units can take sever punishment. A unit of Tactical Marines can take a lot more punishment than a Fire Warrior, until recently I took two ten man squads of FW's and used them quite well as mobile fire support but I always knew that I had to be careful with them. Now I use minimum squads because I needed to find the points for more Plasma and that is the thing with FW's they are OK but not good enough to lose Kroot or Markerlights for (I would have had to lose some of either of these if I had not minimised the FW squads).
Fire Warriors need Devilfish or really good cover to be viable, preferably they should be in Devilfish, they certainly do not work when lined up in static lines waiting for rapid fire weaponry, assault squads and templates of doom to drop on their TGH3 heads. No gunline is not viable, the only line of troops you should have are Kroot.