A common argument against genetic engineering seems to be a perceived danger that a genetically engineered organism will escape into the wild and run rampant, smothering forests and other natural habitats.
However, this seems to be very unlikely to me. If such a successful organism were possible, evolution would have found it already.
If some species is going to spread rapidly, it is going to have to be in a niche that hasn't existed on evolutionary timescales. For example, suppose it is possible to have an organism which lives on road asphalt (extracting its energy from that material and in the process breaking it down) or concrete. If such an organism is possible it would likely take evolution tens or hundreds of thousands of years to find it (by which time, hopefully we will not be so reliant on it for our transportation needs).
But when genetic engineering is thrown into the mix (even if no organism is deliberately designed to eat roads) it is as if we are creating a whole new way of evolving (essentially genetic mutations that change many things at once and don't destroy the viability of the organism). The consequences seem likely to be a drastic increase in evolutionary speed, and consequently the more rapid filling of niches by genetically engineered organisms and their descendants.
It seems to me that it isn't the forests that we should be worried about with genetic engineering - it's the roads.