Generally speaking, there are three things that can make a team annoying beyond belief:

1. The fight last FOREVER, if neither side is significantly stronger than others. Win or lose, if every fight take the full 10 round instead of the regular 3-5, it's going to get annoying. This is especially true in sage world battlefield where time is of the essence. Water and Earth are usually the "problem" here, as water can heal lots and earth can shield lots.

2. Non-interactive. This is where pretty much nothing you do significantly impact the outcome of the battle. Again, whether you win or lose in the end is not the important part, but it's about how the battle plays out. Water immortal, for example, have so many heal+cleanse that you can't really cc them. So all you can do, essentially is pile damage on them while they heal. By the same token, Earth team with gakido and jigokudo c*so be quite annoying due to debuff limitations. Fire main can be somewhat annoying in this aspect as they can make some full CC or counter-CC teams.

3. Relatively overpowered. To be exact, this doesn't mean that the team is annoying because it's strong. But rather that it is strong enough that everyone use it. This is where the term cancer come about, the fact that it spreads quickly. As it were, many of such teams also share point #2, as a team that cannot be counter-acted is more likely to succeed whereas a team that is easily taken apart would fail frequently no matter how strong it can be theoretically speaking.

So no, fire teams aren't cancer right now. There aren't any common builds and only a few fit #2 and none of them #1. In the days of yore, Earth was the most annoying despite fire is the strongest, because of #1--there are Earth team that last 10 rounds almost every time. Right now, the chief cancer is water immortal, mabuti-tenten-mei and RoW. RoW is phasing out and is not as bad due to it is not type 1, but it is very much type 2 and thus gets on people's nerves nevertheless. (Current RoW users are mostly DPH users, so that causes grief in and of itself.