From the Chinese perspective, it's "dangerous" because high investment for so many years, particularly when driven by state incentives or by state-owned firms might induce mis-allocation of resources, high indebtness levels and excess capacity (some might say even voluntary, for example dumping in steel to hurt developed countries' industries). For instance, this paper states that:
The main objective of management in Chinese state-owned and state-
dominated firms has been to maximise not profits but the growth of investment and output. The reward to such bureaucrats takes the form of prestige, power and the accompanying perks of commanding an organisation; the reward being greater the larger the organisation.
Another reason is that investment is more volatile than consumption. However, if investment is supported importantly by state-owned enterprises, this might not apply well to China, which by the way have not had a recession in decades).
From a western perspective, high investment (and low consumption) contributes to a high trade balance surplus and therefore to a high current account surplus, which might be good for the central bank of China (accumulation of reserves) aiming to keep the Yuan relatively low to foster the competitiveness of the export sector, but is bad news for the rest of the world. The latter would benefit from higher consumption of the Chinese population (via Chinese imports and rest of the world's exports). The same story is told of Germany, which would help other countries (mainly European partners, because of same currency) by rebalancing its economy from exports toward more consumption.
Additionally, note that some economists have blamed China (among others) for the Great Recession (e.g. here or here), partly because of their voracious reserve accumulation policies, a core element in the export-led Chinese development strategy. Thus, a more rebalanced global economy might be good for the world economy as a whole, decreasing the likelihood of another recession.