This is just not true. There are 3 major subtypes of opioid receptor, and various drugs have substantially different selectivity ratios for each (I.e. some drug

This is just not true. There are 3 major subtypes of opioid receptor, and various drugs have substantially different selectivity ratios for each (I.e. some drugs are different re: sedation, respiratory depression, psychosis). Even analgesia, the prototypical mu effect, is also a delta effect and the two might be different for acute vs chronic pain. As neuropharmacology and biology remain fields of active investigation, there are several other putative opioid receptors which aren't characterized very well at all but probably have some relevance. Diverse multiple-receptor heteromers are beginning to demonstrate pharmacological relevance (e.g. different binding profiles at mu-delta heteromers) and also at other protein-protein interaction sites (e.g. some drugs inhibit arrestin recruitment and activity dependent receptor internalization). Also the pharmacokinetics/dynamics matter, things like the central/peripheral availability (e.g. how much constipation) and speed of availability to CNS. Metabolites may be psychoactive or may affect metabolism of the primary drug. Off-target activity is present in everything and may be meaningful especially in some analogues (e.g. pethidine has significant dopamine transporter inhibiton) or in plant-derived morphine/heroin which comprise a complex mixture of compounds with diverse activities). All of these things color the subjective experience of a drug. There is not much study made of subjective qualia because the models to describe this sort of thing are not very nuanced or descriptive, but more because it is not very relevant to clinical clinical practice. However, people who use tons of these drugs all the time do report subjective differences along dimensions such as "speediness", sedation, euphoria, "rushiness", malaise, therapeutic index, timecourse, etc. For example a user might report "Compared to heroin, oxycodone feels more energetic, while fentanyl has a nice initial rush but is much more sedating and wears off quicker", stuff like that. TL;DR in vitro mu-opioid activity is not sufficient to explain objective or subjective variance between opioid drugs, and like much other biology there is much work to be done for all the nuance to be well understood.