I think the issue is individual variation. Not everybody is going to be able to take MDMA every weekend for years with no perceivable negative effects. So its really something that should be played by ear, taken on a case by case basis.
My advice is that if you notice you're starting to get harsher comedowns/hangovers, find another drug
As far as the OP's question about mephedrone
https://www.ncbi.nlm.nih.gov/pubmed/27908258 - "36 papers were suitable for the review. Neurotoxic effect of mephedrone on 5-HT and DA systems remains controversial. Although some studies in animal models reported no damage to DA nerve endings in the striatum and no significant changes in brain monoamine levels, some others suggested a rapid reduction in 5-HT and DA transporter function.
Persistent serotonergic deficits were observed after binge like treatment in a warm environment and in both serotonergic and dopaminergic nerve endings at high ambient temperature. Oxidative stress cytotoxicity and an increase in frontal cortex lipid peroxidation were also reported. In vitro cytotoxic properties were also observed, suggesting that mephedrone may act as a reductant agent and can also determine changes in mitochondrial respiration.
However, due to the differences in the design of the experiments, including temperature and animal model used, the results are difficult to compare. Further studies on toxicology and pharmacology of mephedrone are therefore necessary to establish an appropriate treatment for substance abuse and eventual consequences for public health."
I don't think there are clear results but it has the potential to be a serotonin and dopamine neurotoxin in animals. I would err on the side of caution and treat it like you would methamphetamine. Chronic meth users have an increased risk of mental illness and Parkinson's disease - I think the case with drugs that induce a subtle neurotoxic effect that is cumulative over time is that when the neurotoxicity occurs slowly enough, the brain can compensate and you don't get above-threshold symptoms until you reach a critical mass.
For example, the vast majority of dopamine neurons have to die off before you become Parkinsonian. One of the reasons for that is that if the cells die off slowly over time, the brain has time to compensate. So who knows how these drugs will affect you when you are 70 or 80. An extraordinary amount of people get a neuropsychiatric or neurodegenerative disease at some point in their lives psychostimulants notwithstanding, so we really don't have good data about what these drugs do acutely in humans and what the long term consequences will be.
There could also be a lot of consequences that are applicable to any drug that tends to be binged upon with subsequent sleep deprivation.