it doesn't generate much in terms of particulate emissions (eg it's not dirty like diesel) but it's quite bad in terms of greenhouse effects. the natural gas pipeline network leaks a relatively large amount of the gas that transits it (it's on the order of several percent per year) and because it's such a distributed problem it's not economical for companies to fix it purely on a cost-savings basis. And natural gas (methane) is an extremely potent greenhouse gas - it is #2, behind only carbon dioxide in overall impact on the atmosphere, and something around 100x as potent as CO2 over a 100 year lifespan.

https://www.cnbc.com/2018/07/06/the-us-natural-gas-industry-...

new studies suggest the leak rate is around 2.3%, which is about twice as high as previously predicted - and at 3% you have offset all the benefits compared to coal generation. Again, obviously that's still better in terms of particulate emissions/acid rain, but you wouldn't be doing any less global warming. So it is still significantly bad in terms of greenhouse effect, not nearly the panacea the fossil-fuel industry would like to present it as.

(and conversely - the fact that it's such a distributed problem makes it difficult/expensive to clean up. you can capture CO2 and particulates from a point source - it's expensive and it sucks but you can do it. But you can't stop 2 million miles of pipes from leaking a little bit at every fitting.)

the only saving grace is that methane does oxidize relatively quickly, on the order of tens of years, whereas CO2 is pretty much there forever. but if we're still pumping out more natural gas into the atmosphere 10 years later, that's not really the fix people think it is. Again, it's 100x more potent over a 100 year lifespan than CO2, the "long term" picture isn't all that rosy either.