the reason these drugs aren't very popular when sold as "bath salts" is that the labeling, packaging, etc is so low grade, there's no quality control, it's often cut with all sorts of fillers, and the active chemical can vary even between 2 packages of the same "brand". plus there have now been lots of bad press about them.
generally people who do these drugs try to get them in pure-ish form from research chemical vendors, so you have some clue of what you're getting & can dose it effectively. mephedrone, MDPV, and methylone were the top 3 active ingredients, although they are banned almost everywhere now.
mephedrone and methylone do have some effect on serotonin release so can cause hallucinations at higher doses. MDPV is a straight up stimulant but it can make people go into delusions and have massive panic attacks, etc. there are many reports of people taking "bath salts" and then going on rampages or attacking police officers, sometimes they harm or kill themselves without being fully aware of what they're doing either.
some people find that MDPV especially is not really euphoric the same way cocaine or methamphetamine is, it just leaves them getting really spun out and yet wanting to take more.
these drugs do also cause people to be sleep deprived and can put stress on your body if you're not eating properly. they're actually quite risky when sold as "bath salts" so that's why people advise against buying/using them that way. besides you don't generally want to support the "companies" selling them financially.