This is a discussion i had with a couple of friends (both involved as caregivers, wich is not my case). So the approach of the exchange was 100% empiric in both side. Take it as it.
The fact is that the equation appear to totally change depending on the "output" aimed : weed grade or seed grade. Even with close and experienced friends it can be difficult to reduce the debate to its simplest and coldest expression. Both sides can't avoid reccurent patterns of experiences that sometimes are opposites in results.
For the weed grades, we have reached a shared consensus on the fact that a plant can have multiple apogees and perigees during its artificial "neverending spanlife" (cannabis stay an annual by nature). It can be annoying for a selection and the grade constancy in the exact same way for somes specimens. We sorted too many different cases to been able to generate a kind of absolute rule. But we agreed both sides that it depend on the genotype of the specimens in question and that the "stabilized lines" (mostly old school today) handle it more in ease that with "neverending heterosis loops projects".
For the seeds grade (genetic side, not for oil etc...), it's a problematic that somes "long term" breeders have faced at least one time in theyr life. And it's generally when you start to explain the details and methods that tensions grow up lol Outside the maturity of the clones used and the right timing to respect for pollenisation, the conditions of the initial specimens can change the expression of the offspring from a batch to another. Sometimes it's usefull to lock a genetic, sometimes it's just a pain in the (...) with bitchy cuts.
In very synthetic, we were able to say in one voice that irreversible genetic drifts are not linked with the specific act of cloning but with the intrinsic health of the plant (=donor).