It depends on what the DoS is targeting. If hashing is being done with an expensive hash function you can absolutely cause a lot of resource usage (CPU or memory depending on the hash) by sending long passwords. That being said this likely isn't a huge concern because only the first round needs to process the whole submitted data, the later rounds only work on the previous round's output.

Simple empty requests or connection opening attempts are likely to be stopped by the edge services such as a CDN and fleet of caches which are often over-provisioned. A targeted DoS attack may find more success by crafting requests that make it through this layer and hit something that isn't so overprovisioned.

So yes, many DoS attacks are request or bandwidth floods but this is because they are generic attacks that work on many targets. But that doesn't mean that all DoS attacks work this way. The best attacks target specific weaknesses in the the target rather than pure brute-force floods.

Very true and a good explanation of DDoS but I was talking about DoS generally, not specifically DDoS. In my (admittedly pretty limited) experience, a single mega request which is not blocked or rejected by your server can cause it to choke. If you don't have sufficient redundancy or if you get several of these requests coming through it can take down some of your backend services.

It's a good point though, there are lots of different attack vectors each fun in their own way that you need to watch out for.