The problem is technology. Here's a Serbian perspective:

Huawei's Safe Cities project uses Chinese-owned and operated facial recognition technology, in cooperation with the Serbian Ministry of Interior, aggregating data from a hundred thousands of 4K security cameras around public places in Belgrade, and enabling tracking by person, and not by place and time. It is infeasible to disable this system and use a domestic one instead. China has complete remote control.

Chinese CMEC (China Machinery Engineering Corporation) and Sinomach in general produce and maintain hardware for some of Serbia's coal-powered thermoelectric power plants, and CMEC is building a new power plant block right now. The control software is provided and maintained by Chinese companies. It is infeasible to disable theses systems and use domestic ones instead. China has complete remote control.

Serbian national railway companies (it was split into multiple ones, including Serbia Train (Srbija Voz) that operates trains and Serbian Railways Infrastructure (IŽS - Infrastrukture Železnica Srbije) that operates the railway infrastructure) are partnered and deeply integrated with CCECC (China Civil Engineering Construction Corporation), CRCC (China Railway Construction Corporation) and CRI (China Railway International). The trains are being imported from China, the rail is being reconstructed by Chinese workers, there are Chinese flags next to the rail for it's whole length. The system will be reconstructed with Chinese technology and deeply integrated into proprietary systems. It is infeasible to disable theses systems and use domestic ones instead. China has complete remote control.

This is happening also to Železara Smederevo and RTB Bor, and more is on the way. Chinese military aircraft is planned to replace old Soviet and Russian planes, and Serbia has already imported Chinese UAVs and anti-aircraft weapons, as well as tanks and advanced multipurpose vehicles. Huawei equipment is installed in all cell carriers and powers intenal networks for military use as well as civil interconnections with ISPs both domestic and international, through the Internet Exchanges in Belgrade. More things are planned.

The only things that could be seized, if a conflict were to occur, are the roads and the dumb hardware. That is not useful, but in any case a conflict will never occur because it would be suicidal for a country so deeply integrated with China to try and pick a fight with such an economic and technological superpower. The problem is that this relationship is not symbiotic, it is dangerous for Serbia due to Chinese tendency to ignore environmental laws, manipulate reports and conceal information about accidents or toxic waste, as well as the planned extraction of natural resources that is one of the key points of the Belt and Road initiative.

EDIT: Oh and one thing, before someone calls me sinophobic[1]: It would be equally bad for Serbia if any other country did such a thing. But they didn't, not even Russia had ambitions to make this a puppet state of theirs.

[1] Of course, for all intents and purposes, in this context, China === Chinese Communist Party with the Paramount Leader Xi Jinping, not the Chinese citizens or people of Chinese etnicity.