First of all, the new pipeline will take many years to build, and of its capital cost of $55 billion, some $30 billion is to be raised by Gazprom. This will strain both Gazprom and the Russian treasury and limit what other projects and mischief they can undertake.
Moreover, the inevitable delays and corruption that attend such projects will test and may sour the Russia-China relationship. Similarly, Russia is inclined to use energy exports as a political weapon and tends to be heavy handed about it. This is likely to antagonize the increasingly proud and prickly Chinese.
Finally, the project will supplant oil that China now imports from petrostates in the Mid East. Since Iran is first on that list, the new pipeline does not token well for the Iranian regime.
Also considering the US will soon be a net exporter of oil, the entire OPEC organization is going to be seeing demand heading south with a bullet. Which just breaks my little heart. /s