It's easy for those of us with more experience and/or more detailed knowledge of biology and/or chemistry to say "Use product XYZ" or "Do ABC, not DEF." because we have a deeper information base on which to draw....
If I hadn't done my master's thesis in substrates for growing aquatic plants then I would still be leery of adding a rich terrestrial fertilizer to my tank.
It nice to see people who actually studied in this field in academia participating on this board regularly. My PHd was in Physical and Organometallic chemistry. My publications are on Researchgate, they are in Organometallic synthesis and Physical Chemistry particularly SSNMR (J. Geller et. al McGill). Did you by any chance publish anything during your Masters or beyond?, I'd be interested in reading your research on substrates if you have anything you would like to share.
I agree with you, Osmocote is the superior overall nutrient supplement; particularly for high-demand systems. It's also very rich and can cause significant problems if used improperly or when something goes wrong.
I don't understand the speed of release of Osmocote Plus (gel cap) versus Fluorish tabs, versus Thrive Caps for that matter or comparatively how efficient tabs are used by plants versus column dosing. Do you have any specific information on that or research studies or guidelines?
How do you know how much the plant is uptaking, how do you control the release?
What do you expect to see in plants that are showing a toxicity from root tabs?
The Flourish Tab + Flourish liquid combo has worked well for me too in "low tech" low demand systems. It all depends on the needs of the system. If nutrient demand is modest and the plants are doing well with the macros they get from other sources, the combination of Flourish Tabs and Flourish liquid can work very well to supplement minor and micro nutrient needs.
Both Fluorish and Fluorish tabs are significant sources of Micros not Macros
. Just reading even the commentary from Seachem on the Fluorish liquid bottle confirms this.
Fluorish liquid has even less Nitrogen in it than the tabs 0.07%!, and that is why you then have to dose Fluorish Nitrogen and Phosphorus if using the Seachem system.
To distill the answer down; even experienced people use them because they work.
The videos I watch are dominated by aquascapers sponsored by Seachem, I am sure many people use it but I wonder if they would continue to do so if they understood how diluted and expensive comparatively the whole product line is.
If you can grow your tank with 1ppm N every two weeks and that is enough, your problem would be potential overdosing not underdosing as long as you knew that, any fertilizer should work.
At least Seachem doesn't hide the ingredients used to prepare like other brands like Tropica. Seachem Nitrogen is KNO3 and salts including Urea which by looking at N:K is like ~60% Urea/Ammonia and ~40% Potassium Nitrate if I did my math correctly. When you dose lean it seems important that the plants uptake what little Nitrogen their is efficiently so using reduced N as opposed to Nitrates is important as long as you don't add too much Ammonia to cause Bacteria or Algae Bloom.
If they didn't work then folks who know what they're doing wouldn't use them. It's also about a system's needs. Sometimes a system needs a high nutrient additive and sometimes it doesn't. In the end, it all comes down to the individual aquarist's needs and comfort level with a given product. "Sometimes we feel like a nut, sometimes we don't.".
I struggle with this often. My LFS owner who I trust is convinced that its important to add Seachem Stability to prevent algae and diatoms at the beginning of a tank setup. At startup he says even with cycled media from a well established filter, he still would add Stability initially and for the first few months. Even after years he adds stability to his shrimp tanks for "strengthening" the bacteria monthly. I have no idea if this is by tradition or if there is any benefit to it whatsoever.
If someone who I didn't know tried to sell me Seachem stability the Scientist in me would say, I already have sufficient bacteria in my filter why would I add dormant spores to my tank, bacteria will grow rapidly without adding something much more dilute than what is already in the filter.
The hobbyist thinking however might be is something like, "Well this guy knows what he is doing and he reccomends it and I don't want to take a chance with Algae so I'll buy the 10 dollar bottle of bacteria even though I will never know if it helped or not".