First off, excellent idea for a thread! More discussions like this around here are sorely needed.
Broadly speaking, I would agree with Lieberman. In hindsight we can see that the China of 1800 was in for some rough times ahead, but we have to look at what the Qing had acheived by that point.
1: With the final defeat of the Steppe Nomads in the 1750's, Qing China had defeated the most significant threat to China for previous millennia. The relatively flat and open geography of Northern China at least had always left her vulnerable to nomadic horsemen from the Steppe, in a way that forested Northern Russia never quite could be. While it should be noted that this was not an achievement unique to the Qing, and fits into a wider story of nomad subjugation in this part of Eurasia, I still think that in terms of Chinese security this achievement cannot be overrated.
2: Economically, Qing China was a powerhouse in 1800. If Kenneth Pomeranz and his supporters are to be believed, China was not only home to the world's largest economy by quite some margin, but was also home to some of the world's most productive per-capita regions. Although certain regions of Europe were more industrially innovative (You know, like Manchester. Or Birmingham. Certainly not "Europe" as a whole), for much of the 19th century China may well have been keeping pace in per capita terms with Europe. And of course in macroeconomic terms China was in better shape in many respects. China's booming exports were draining Europe of her bullion, and the British had to resort to selling illegal drugs to resolve the trade deficit.
3: Size. I mean, have you seen the size of Qing China on a map? That is one impressive Empire! And China's population probably grew swifter than the world average in the 18th century, partially thanks to the arrival of New World crops but also due to Qing agricultural policies which actively attempted to disseminate new agricultural techniques for farmers, as well as refine systems in place for the relief of famine.
I think there is a really good case to be made that Qing China was one of the most powerful nations of the Early Modern Era. I mean, they really taught the Dutch a lesson, didn't they?