Indeed, they'll likely make a mess if they get involved around Taiwan, not to mention the possible political problems that can come from it. But I wonder if they could aid China in perhaps deterring or standing off US and their allies with their much vaunted submarines? Anything that could free up some of China's navy, and make US rethink their strategy?
Russia's greatest assets to China right now are:
#1. Being non-hostile.
#2. Being a potential provider for food and energy.
#3. Providing an implicit nuclear umbrella.
All three are made possible by the Ukraine war. Thanks to the open hostility between Russia and the west, in any potential confrontation between China and US, US has to taken Russia's vast nuclear stockpile into their strategic consideration. Because should China and US go MAD with each other, Russia would be the sole remaining nuclear superpower. Would US really want that outcome? And the thing is, Russia didn't even have to commit anything for China. The implicit nuclear umbrella is there due to the current geopolitical climate. Frankly, Russia would be better off not getting directly involved in US/China confrontation. With their outdated equipment and poorly trained force, the Russians are probably more of a liability than an actual asset.