In general, no. Most "slow" hashing schemes are designed to be memory-hard and highly serial (more threads = more contention, not throughput). Some of the earlier approaches just try to maximize computational requirements, but modern research acknowledges the march of computing power makes that an inherently poor choice for long term sensitive data. Argon2 is just one of the more advanced versions of a memory-hard scheme, building on prior research. I could try to give a poor explanation, but instead I'll link you the spec which is very readable: https://github.com/P-H-C/phc-winner-argon2/blob/master/argon...

As per DOS: rate limit per ip and account, and it's not an issue.

They have to and do use tricks, they aren't just slow. You could trivially make sha256 slow by just applying it a million times (that's not even parallelizable as well), but scrypt et al improve on that by making the algorithm also require a lot of RAM, use complex instructions (hard to recreate on a GPU or FPGA) and so on.

It's really hard to do very well, look at cryptocurrencies trying to be ASIC resistant: They want every normal person to be able to mine it, and don't want companies to be able to build a machine that can eclipse the mining power of everyone which means centralization.

It is a good DOS target, you should definitely rate limit logins to mitigate that. Other strategies are requiring captchas for login or letting the client execute the hash function and just doing a simple (fixed time) comparison on the server side.