This is a question designers, manufacturers and the marketers of most current units hate to answer. Because the simple fact is they don’t know the answer.
The correct question to really help get your head around how complicated it all is - is this …
Can two different machines give identical results?
The logical answer - taking into account the complicated nature of physics and physiology - is of course no. It would actually be impossible. This idea is not just mine, but is bound in law. Once Medical Certification (MDD) of a machine is applied for, not even a single design feature can be changed due to the conclusion this would also change its overall function. If a company is caught doing this it can be stripped of its certification.
So the question then goes to: What results would differ and can those differences be pinpointed, explored and then magnified? Creating units that are hyper-oriented towards one results-based function.
At the moment the industry is in a phase where companies are producing machines that resemble swiss army knives - lots of functions (wide range of amplitudes and speeds etc …) - but like most multi-tasking knives it will always only be good for the small jobs. You wont be killing a crocodile with the blade, and the saw wont go through a decent house plant let alone a whole tree.
You get the idea.
Most of these companies run on fear - fear of losing a sale. The only difference they want to discuss is price, or simple specs that can be put on an advert. It is not in their best interest to discuss such things as the science behind the results, as this is where they will lose your confidence and then the sale.
Note: The price of a machine should not only reflect the quality of the engineering but also the knowledge used to design it. If all a company can tell you is the advertised specs which is identical to a cheaper model then why buy it?
I am not expecting all companies to fully understand this science. As an industry it will be a long time before that happens if it is possible at all. But I do not agree with them covering up their lack of knowledge with the lie that their machines is the same as others - knowing it to be impossible.