That's a good question to ask. I've had the opportunity to visit Seachem's facility and have detailed conversations with their scientists on multiple occasions and feel confident in my recommendation. Flourish Tabs and Flourish liquid are two products designed for both the average person wanting a couple plants in their tank as well as the beginner plant hobbyist who's starting to get into the hobby more seriously. Recommending the one or the other method is the best way for the general populace who keep a couple hardy plants but don't necessarily have systems that need more nutrient input. Once someone starts adding enough plants to need a bit more input using both products is a solid first step.
In my experience, substrate supplementation is a good long-term/slow release base for nutrient supplementation but it doesn't always meet all needs when plant mass is heavy. Having a way to give a little extra via the water column as needed has helped me a lot in the past; particularly in low tech systems. Using the substrate as the main source of nutrients is something I feel is very helpful for beginners. It's easy to understand and is much less prone to mistakes while getting a grasp on the fundamentals of aquatic horticulture. Once that foundation's in place and the person has a stronger grasp on how his/her plants grow and the system itself works, switching to water column supplementation and/or moving on to more complex things like learning how to manage multiple liquids or dry nutrients is more practical. Until that point simple, manageable, and effective is best. Too often in this hobby people who don't really know what's going on and have just enough information to be dangerous tell beginners things like they have to use more light than they're ready for, high CO2, and a dosing regimen like EI. Almost as often, I've seen those people have major issues, fail because they don't have a solid foundation, and leave the hobby. Helping people have success in the hobby is my goal, not pushing them into something they may not be ready for.
I agree with much of what you have said, although I don't think using root tabs is plant mass dependent. If that is in fact what your saying.
Just so you know I've been in the hobby for 13 years and have taken it further by professionally setting up tanks at both residential, commercial and LFS locations on the side since I enjoyed it. So I am not a beginner or someone with just enough information to be dangerous. Everything is in context. For a newbie the simplest approach to me is to simply dose the water column and not confusing things with root tabs which can also have a negative effect if the new hobbyist isn't religious with his maintenance and the tabs disperse into the water column. The upside to me is simply not worth it.
The problem with some forum advise IMO is sometimes people follow experienced people who can do things that others can't and they fail. Experienced people can grow things without co2, with less light, etc. but the general masses will fail. They fail for the main reason they are not as dedicated to their tanks as the people giving the advise. So to use strong lighting, this or that isn't going to work for them since the maintenance and dedication goes hand in hand with that. I try to give practical advise on what works for most plants and most setups. There's always an exception or a unique set of circumstances that might go against this.
Another problem I see is that people get turned off thinking they need an advanced degree in chemistry, etc to succeed. There are many here that bring their paper credentials and think it's always applicable to actually running a tank. Many here preach their chemistry knowledge etc and don't even have tanks. Some of the most successful people in the hobby come from a fine arts background. Amano (RIP), Knott come to mind. Actual experience will always trump reading or a degree IMO.