it depends on what you subscribe to.
I don't believe in the golden triangle, I think it is unsafe, founded on planning for the best, and not in touch with reality. Proven by personally having my regulator removed from my mouth on multiple occasions. I subscribe to primary donate where my primary second stage is on a longer hose and is the hose I give to someone if they ask for it, but if they take it, well they're going to get it anyone. My secondary second stage is on a very short hose and is on a necklace or suicide strap around my neck. I can get to it without using my hands if I have to, but it is always within easy reach and I never need to wonder where it went.
Now, I prefer to have my second stages match because now it isn't my buddy that has the cheap octo so as long as they can breathe it doesn't matter, it's ME that has the cheap reg and now I have to deal with breathing on it. So no, I disagree with your LDS in their assessment and think that that is an unsafe mentality. I want the potentially panicked diver to have the easiest breathing reg as possible, and I selfishly also want to have the best breathing reg possible.
We have to step back a bit and understand what balanced and unbalanced second stages do as well as the overbalanced first stage. What the sealing diaphragm does to a first stage is increase the IP at some percentage over ambient as you go down. I.e. IP is say 130psi at the surface so you expect it to always be 130psi over ambient pressure regardless of tank pressure and regardless of depth. The overbalancing actually increases that to say 133psi over ambient at 100ft, and so on and so forth. The actual percentages depend on the size differential of the diaphragm itself, the sealing mechanism, and the bit that connects them. I don't know what the delta is on the Legend so don't quote the numbers, but the principal applies to all over-balanced regs. Think thumb tack, big silicone environmental seal, smaller diaphragm. The big seal tanks a total amount of pressure from the water and transmits it onto a smaller area, so the pressure increases. This is not in fact a "feature" it is a design consequence that has been turned into a marketing scheme. Overbalancing is also a little bizarre because it's actually over-compensation since it is due to depth, not tank pressure.
A normal second stage uses a spring to keep the seat closed and the IP of the regulator "fights" the spring. I.e. the spring may put 150psi on the seat, and the IP fights back at 130psi, so your inhalation has to overcome the 20psi differential. They market the overbalancing with as you go deeper that differential gets smaller so the inhalation is easier, but at some point the tables turn and the regs freeflow. It's VERY deep, nowhere near where you'd be going, and nowhere near where all but a handful of divers go. I say normal above because there are various designs of second stages, but the ones you purchased work like that and that is the "normal" design that most regs use. Poseidon being the notable exception to the rule.
Unbalanced regulators have a set spring pressure and if the IP goes up, it gets easier to breathe until it starts freeflowing, and if the IP goes down, it becomes harder to breathe. Any change in IP will be experienced by the diver. Differences of a few PSI are not noticeable for reference, so don't get hung up on that.
Balanced second stages use the IP as part of the "spring" mechanism and are able to compensate for variations in IP. They are particularly useful when paired with an unbalanced first stage or a balanced first stage with imperfect balancing since there will not be a significant change in the WoB. With a modern, well balanced, well maintained first stage, the balancing mechanisms are not required, increase failure points, and don't provide a meaningful benefit to the diver. Most high performance second stages are balanced, and it's not something I'd run away from, just not something that I'd necessarily pay more for. There are other features on the second stages such as metal barrels and heat sinks that typically only come on balanced second stages that are requirements for me in cold water diving.
So, long story short, I dive with unbalanced second stages exclusively, Poseidon Jetstreams, and have no problem with them. I do not typically recommend those regulators to those divers that are staying primarily in the US as I believe there are better value options out there, however I would not return anything if you are happy with the price you paid, and the service you received. You have a very good regulator set that will last you as long as you take care of it.
