I don't think this is hard to understand. The newly certed zero-to-hero OWSI probably isn't great and can just about teach you to dive. 20 years later and the same person might well be a tec instructor with a fantastic reputation or they might have got a real job.
If you want to do a speciality then the key thing is to look for a person that understands what that speciality involves. Open water sidemount is a fad, driven by the industry to sell kit and sell courses. A solution looking for a problem. The "problem" is usually a confined entry/exit point to another part of a dive which is 99.9% likely to be in an overhead environment, wreck or cave. Therefore sidemount traditionally has been a technique for overhead dives and as such is what we now call "technical" (whatever that is).
In cave training you do cave before you do sidemount. So the tighter passages are off limits in the early part of your progression. One advantage of a sidemount rig is porterage - you can carry each can separately rather than a twinset. Cave entrances are not always next to a car park. So if you wish to undertake cave training in SM some agencies will let you, but it is their call whether they recognise another agency's cert or not.
PADI's SM course is part of the US driven fad and I have no doubt it is good enough for what it is intended to do. It is not integrated with overhead training as PADI doesn't do overhead training. If a person wants SM for non overhead use for some reason (back injury is a good example) then the PADI course almost certainly will do the job.
This tribal loyalty to one training regime or another is bonkers. Pick what is appropriate for your personal circumstances and accept that other people's needs might just be different. Accept that they are all commercial organisations and so will want you to train with the people that pay into their coffers - the affiliated instructors.
But no matter, I think the basis of what you're saying is correct ie choose an instructor that's capable of teaching you what you need to know, regardless of agency.
But that still begs the question, re the OP, why try and short cut the course, what benefit us the extra bit of plastic providing?
PADI's a recreational outfit geared to making money from every side. Nowt wrong with that.
Problem is where PADI instructors are just instructors. They don't have to be subject matter experts, living and breathing their "specialties". Most of their instructors -- and assistants -- spend their time dealing with novices, not doing advanced diving. So if they're not "doing it" all the time-- as is the case with all the other specialist technical diving agencies -- how can they expect to be on the top of their game.
Of course there will be exceptions. But as a whole, if you want technical or specialist training, you're much better off going to those who do this all the time.
PADI don't do tech courses, so yes you would need to go with an instructor qualified with an agency that does if you wanted to go down the tech route.
The PADI and TDI/SDI SM courses both have the same aim, to teach the entry level diver an alternative dive rig.
In terms of recreational diving the agencies are all equivalent.
As has been said so many times, choose the instructor, they're the ones that deliver the course. The agency that provides the bit of plastic is secondary.
Er, yes they do
Granted not all PADI instructors can teach these and the recommendation to pick the instructor for the course / type of diving and not the agnecy is still the best advice, but as an agency PADi do Tech.
I personally prefer my SM to my BM, however the majority of my dives are made in BM with a couple of small slung cylinders because logistically it's easier. (Normally we're on a 2 day trip where you need to bring all the gas you're going to use with no refill options - I can do it in SM with transfils but its a faff)
I can (and do) take my SM all over the world, and it's easy to chuck teh slings on a couple of cylinders and hey presto. Try that with manifolded doubles - unless you're off to specific "Tec" dive centres. My SM cylinders are easier to transport (as singles) and lug about. I can switch between steels and Ali's as appropriate without having anything dedicated.
There are of course pro's and cons with all configs, but I've yet to come across a situation in my diving where SM would be better than BM doubles
Back to the original question. I agree SM is one of those things where the instructor needs to be an expert in the system of your choosing. I could on paper teach SM, I choose not too because I only know my system (X-Deep Stealth 2). I have and still do, seek advice and opinions form a number of experienced sources, because Sm is one of those things that gets customised and perfected to the type of diving you do (personally for instance I'm a fan of ring bungees).
With all Tech it really is worth finding an instructor that has a decent amount of experience, and certainly extensive knowledge in the subject, rather than choosing an agency