Buffer is bad precisely because it increases the lag between having a gap in provision, and that gap being obvious to the player.

With no buffering, as soon as your demand for steel is greater than than production of steel, then the bottleneck is immediate and obvious. The solution is also immediate and obvious: Build more steel.

Buffering, in particular belt buffering, and in particular busses, contribute to mask this issue. There can be a great delay between increasing consumption above production, and so the root cause can be very hidden. It may also be that the ultimate root cause is that steel production is low because it's limited on how much iron ore it gets. If everything is bussed, then it can be hours before resource constraints are hit, by which point it's very hard to see what's happened to cause the shortage, and also by which time the factory may have expanded further.

It also constributes to see-saw production, where-by a shortage in one area causes a pause, which allievates the root cause shortage for a while by backing up other production. The longer the lag between cause and effect, the greater the banding effect, further masking the root cause.

A bus also encourages bottom-up, which further encourages massive over-consumption of base resources. If you start building green chip production and bussing it, the bus may will fill and buffer, making it look like you've got plenty of green chips. In turn, as the green chip production stops, it'll look like you've got plenty of iron plates. You'll build all your malls and other production, satisfied as you build each that they run fine and are not over-consuming.

Only when later everything starts to run at once you realise that the stuff down the end of the line is getting scant resources, as previously each part was running in isolation before serious amounts were required.

In contrast, a top-down approach involves building the final result first, then at each step building what's needed to feed it. This ensures that there is always enough provision, and everything can be placed to minimise buffer to reduce lag and improve feedback time on problems. It also reduces pollution since any item on a belt represents inventory for which you've paid a pollution cost but not got any final results from yet.

The spaghetti approach can lead to "under-utilised" buildings, such as smelting array that ends up only needing to supply 0.3 of a belt. But in factorio space is almost endless, and there's little to no cost to idle buildings. The power drain of idle assemblers, particular the bare (no module) level 2 buildings you'll likely be building before end-game, is extremely low.

For late game post-rocket, this changes of course. With beacons and level 3 assemblers with modules, the idle draw is significant, and you may want to optimise ratios and look to eliminate how many assemblers you run idle. ( That said, power is almost non-issue in 2.0 with nuclear power being much easier to run efficiently than previously, so the large solar fields aren't really needed anymore. )

Busses have a strong visual appeal, but unlike "cable management", there's no airflow to consider in factorio. A messy spaghetti base isn't inherently inefficient. It doesn't affect productivity to just run short belts all over.

The visual temptation of the mega-bus is clearly alluring, it looks good on youtube video guides.