If you have broad experience of electronics assembly around the world, in different climates, it would most interesting to hear your experiences of what those environments are like.
Many years I used to work for a company that have own fully equipped SMT assembly floor +/- same size as in EEVblog video and briefly visited another, much bigger one. From environmental control point of view they are all the same: controlled temperature 24/7 all round the year both in assembly and components storage places. No freaking open windows for god's sake. Big factory even have controlled humidity so it does not float around when rain comes.
To put it in Laymans Terms: when your pick&place machine alone costs 10x more than ventilation/condition system for whole factory floor then it is insane stupidity to invest in manufacturing equipment but save on climate control.
In case you wonder why it is so important to have control over temperature & humidity in electronics manufacturing (SMT assembly) floor - it's very important If failure rate of boards that comes out of SMT assembly suddenly becomes high, then QA experts of customer will visit factory. If they do not see that factory follows environmental or ESD safety standards (yes, factory *must* have temp&humidity monitors/loggers all over the place), then lawyers of customer will tear it apart.
You have narrowed "electronics assembly" to a single topic - stuffing PCBs. This is a small part of the overall assembly space in an integrated assembly plant, and these days it is often subcontracted, so it doesn't occupy the final assembly plant at all. What about all the other assembly work? Putting displays, multiple PCBs, connectors and other modules into a final product takes lots of space, and tightly controlling the atmosphere would be a waste of time for most of it, unless the local climate is quite troublesome.
In the assembly plants I have been in, in the UK and Asia, PCB stuffing takes a much smaller space than, say, the final burn in area. Everything beyond goods inward, and up to the case being screwed/glued/welded shut requires a proper static control regime. Burn in is typically something like 48 hours running, followed by probing to ensure everything is functioning to spec. That means burn in happens before the case is closed, to facilitate the post soak probing. You need to manage static all the way through. If you make 10k or 20k TVs a day, that takes a lot of space. The only environmental control worth bothering about during soak testing, for most products, is to avoid a condensing atmosphere and excessive dust buildup.
Environmental control is seldom an all or nothing issue. Most factories control specific areas in specific ways. For example, if there is any paint spraying, that requires a moderate quality clean room to avoid particulates spoiling the finish; it requires efficient capture of waste paint; and if there is the risk of a condensing atmosphere it probably requires humidity control. You don't apply that kind of practice to the whole plant, though. By only applying it in specific areas, the energy costs are kept under control.
The requirements for PCB assembly depend a lot on the climate. In a place like Singapore, where the humidity hovers around condensing for much of the year, humidity control is important throughout an electronics assembly plant. The places where really tight environmental control is needed during PCB assembly, it is ensured by the equipment itself. Most SMD reflow and vapour phase soldering systems are completely enclosed, with a well controlled atmosphere inside. Old through hole wave soldering lines were far less controlled. The use of a dry nitrogen atmosphere within an SMD soldering system is pretty common practice now. Its not expensive to continuously separate nitrogen from the air these days, as long as your requirement is not for perfect purity. That dry inert atmosphere, combined with preheating, helps dry out the components before they reach the high heat of the soldering process. Eliminating most of the oxygen eases the demands on the flux, and results in a less tarnished final product.