With regards to sweetness I haven’t noticed any drop in usability over a period of time (2+ months so far). based on the test samples I have made over that time and the level of sweetness using the 1 drop finger taste test also of just the Stevia base.
With regards to things like harshness, I have found so far it doesn’t really affect the finished flavour with whatever attributes it has, only in a negligible way I have found it just adds a lesser level (that is more pleasant) of sweetness, than sucralose does as I can find sucralose quite sickly. Of course if the concentrate is already loaded with sucralose, then you are fighting a losing battle anyway over the level of sweetness in the mix.
This is only my experience though. If people are saying the level of sweetness is dropping off then you should take it into consideration, The only thing I would say though is without reading these posts myself, are they using the already diluted mineral water based stevia that is shop bought, rather than powder? as the shop bought one wouldn’t be as higher concentration as say a 1g/9ml self mix I use, I would have thought? Whether that has any effect overall I don’t know. As with everything DIY it is trial and error, so you can form your own opinion.
The ingredient that has surprised me the most in removing harshness, sweetness, tart or sour is saline. Used at the right level for the mix it really takes the edge of, say a lime or overly sweet strawberry and just leaves a smooth fruit flavour. It produces a much more interesting finished result for me, than any other additive I have tried so far in 4+ years of mixing, It is just about finding the right level of saline for the mix though as too much and it just mutes the flavour completely. Reminds me of TPA Smooth but has less drastic results and easier to use and has worked on Tart, sour, sweet and citric flavours equally as well.
But I am still at early testing stage, but every time I talk about it, I get quite a few negative comments from people about their perceived effects on liquid and coils that heavily diluted salt may have on it/them. I have yet to personally experience any of these things people say. But I am still in early days of testing it out for myself, so they maybe proved right. But I am going to persevere with it for a few months yet.