The IRT (Institute for Responsible Technology) releases a report on the GMO regulatory system in America.
It's not true that here is no regulatory oversight, but their article for the layman that explains the position summarises the difficulty in one telling paragraph:
"Unfortunately, no meaningful laws or regulations reign in these biological time bombs. Ironically, the Environmental Protection Agency’s regulations that do apply to most genetically engineered microbes treat them explicitly as chemicals. They employ evaluation standards developed for toxic substances. They have yet to create systems to analyze the very different set of risks associated with living microorganisms (Mandel, 2014)"
Is this scare-mongering?
It's probably impossible to know at this stage since the technology has not been long in widespread use.
Nevertheless, the more we discover about our human body the more we understand that our internal microbiome is critical to our health. If that is so for us, then it is so for probably all species on the planet and certainly for the health of our soil our crops and vegetation in general. It has evolved over millions of years and the risk of accidentally releasing anything that may inadvertently destroy important parts of it doesn't really bear thinking about.
Once this genie is out of the bottle it would probably for all practical purposes be impossible to put it back.
The precautionary principle must surely be applied here, and appropriate regulatory regimes introduced to mitigate the possible risks.