Quality varies highly both from vendor to vendor and even batch by batch from the same vendor, that's about all I'm allowed to say about that aspect of the question.
Safety is a hard one to say with any certainty as there can be literally no history of human use when many research chemicals come to market. Even of those that do have some years of use behind them, fatalities can and do still happen; sometimes a straight up overdose, sometimes a fatal combination with other substance(s), and sometimes people die with a research chemical from direct pharmacological action (25x-nbome for example). Theres also always the risk of getting something like bromodragonfly accidentally labeled as something else like 2c-b-fly or 2c-e (both have occurred in recent history).
If you're going to use research chemicals at all, the utmost care should be taken and only after learning everything possible about a given substance, then titrating up the dose slowly, even if you have some idea of what an active dose should be.
Tryptamines, especially psilocybin analogues, are probably close to the safest, with lysergamides likely being fairly safe. Keep in mind that even with those compounds, there is always a degree of risk, and all you can do is minimize it, but always know there is substantial risk with research chemicals, no matter how safe you are about your use.
Dissociatives, opoids, stimulants, and cannabinoids are much riskier and should only be used after much research and in as safe a way as possible, with a sober sitter whenever possible. If you're not familiar at all with these substances or have a way to measure them accurately, avoid them altogether.
Benzodiazepines are a different beast altogether due to both high potency and ability to cause amnesiac effects at just a milligram or two, and withdrawal from benzodiazepines can be fatal. Liquid dilution after measurement with an accurate milligram scale is prerequisite to their safe use.