If not, you might be in trouble -some frames can be made to have the horizontal stiffness of aluminum but with the vertical flex of steel frames, making this one of the more desirable material. At the end, the unwoven spot will fail by breaking. However, some manufactuer will use carbon main frame only with al lugs and rear triangle. CF tends to be flexy at times, however, like any other material, it can be made to be very good. Almost any material you can think of can be built into successful mountain bike frames, providing the engineer who does the design work knows the strengths and weaknesses of the material, how to use and compensate for them, and, most importantly, how to translate those concepts into production processes that build reliable bike frames.
All materials are available in different grades, with stronger grades usually being more expensive.
Stronger steels make for more expensive frames because, while the raw material is relatively cheap, its very strength means that shaping it into tubes, cutting and joining those tubes is more time consuming and hence costly. The ride is more forgiving, but wastes more energy. There have been some very long, tedious and circular discussions of the whole issue of the actual effect of frame flex on efficiency in r.b.tech. Given the calibre of the minds there that have failed to reach meaningful conclusions, I think it's safer for you and me to leave this one alone rather than repeat folk misconceptions. I will say that it's very hard to imagine that, say, Henrik Djernis could have ridden steel frames to three world championships if they wasted any significant amount of energy.