I see where you're coming from, but it's a 48-gallon beginner tank. This would be like planting mint in a container, not a garden, to use your analogy. New hobbyists have to learn like we did. First, how to grow coral, then what grows too fast. Easy enough to manage in a small tank.
I've seen way more hobbyists burn out early and leave the hobby from initially adding challenging coral that die and feed an algae farm, than end up with an overgrown tank they can't manage. So, I will continue to recommend easy, fast growing coral to beginners.
I am honestly not trying to be argumentative; I simply don’t agree in the slightest. I think that the advice for any new reef aquarist to keep those species is extremely short-sighted and countermands much of the logic for providing the advice in the first place.
The garden analogy had nothing to do with the size of the system, but rather the permanence of the decision, and that the act of undoing it means basically starting over. While this may be perfectly fine for some people, or even a stepping stone for others -- just as many people will not want to start over, and the 10, 25 or 40-gallon tank is not a stepping stone, but rather their permanent and only reef.
I personally don’t care what anybody puts in their tank or their garden, as long as it does not affect me. Your concern here about “new hobbyists” failing is exactly the point I am making. Advice should come with context. “Hey, mushrooms and GSP are easy to grow -- but be fully aware that if you do decide to stock them, they will be hard, if not impossible, to get rid of in the future.”
As for the “easy enough to manage in a small tank” sentiment -- that is demonstrably wishful thinking that is more often than not wrong. It ignores the hassle, luck, and expense required to pull it off. Not everyone is willing to pull rock work and bleach it, or cook it in the dark, or hand scrape it. Not everyone has the time or money or interest to replace rock that is infested with unwanted coral fragments, In many (most) systems, doing so is insanely destructive to the rest of the ecosystem and/or aquascape.
So yes, those corals are easy to grow, but advising people to start with them should come with context about the decision, other than "they are easy to grow".
You certainly don't have to agree and can continue giving whatever advice you wish. I just don't happen to agree with it because I think it sets people up for eventual grief if they don't know what they are getting into.