There are a couple of things going on here... one is that after a few cycles the results that you get from a particular compound seem to taper off... i.e. the major effect that compound will have on your genetics will approach it's maximum... which is one of the reasons why you see folks recommending just a test-only cycle for first time AAS users, and advanced cycles that seem to have enough different high dose steroids in them to kill a horse. In the OTC PH/PS/DeS world, if you start off with m-sten/DMZ, where do you go from there?
Another aspect is sides. The idea here is to take the mildest substance that you can for your first time to get your feet wet from a perspective of side-effects. Personally I would recommend hdrol for first time users, it's mild with regards to sides, the strength gains are nice, and size gains are decent... In my experience, EPI is stronger, and would be a good PH to graduate to after using hdrol... although mechabol is another good candidate... in fact, I found it to be even milder than hdrol.
After playing in the shallow end for a bit I suppose I could recommend going to something like m-sten/DMZ unless you want to play with the "tren" based stuff, which I personally don't from a side effect standpoint.
Note that another reason to progress in this manner is to manage the growth/strengthening of your connective tissue. i.e. if you start off with DMZ you'll probably find that half way through your planned cycle you've pulled a tendon, since muscle grows faster than connective tissue. This may still happen with hdrol/mechabol, but is less likely... note this is why I also tend to recommend that folks lift "heavy" (even if that means going down to 1-3 reps) for at least a couple of months prior to a cycle, and to try to increase reps, not weight, when the strength comes. Otherwise, many folks find themselves injured half way through a cycle and not realizing the full gains they could have...
That all said, as far as the "risk" of taking multiple PH over time from a health standpoint goes, I suppose 3 new synthetic molecules that have never been safety tested is worse than 1... but I think if there were any serious short to medium consequences (besides the expected stuff like liver issues in those who abuse dosages and don't know how to keep their liver protected) we would know about it considering how many folks have used these products over the years. If that's really a concern, I'd recommend sticking with legit AAS (test, var, winny, dbol, etc.) as those molecules were developed by big pharma companies for used in clinical medical settings, have been through animal & human trials, etc.. and are still prescribed by Dr.s today. I personally have never crossed over to the dark side just so that I can look my kids in the eye and tell them I never did anything that I didn't buy OTC... even though for my health, legit AAS is clearly better.
Anyhow, maybe not a "science-y" as you were looking for, but hope that helps.