I don’t actually use it but i have in the past.
It’s super expensive compared to other sources.
From my understanding of microbes and there roles, while it certainly want hurt to give them upto harvest. Considering if they been doing there job and providing benefits to the plant, then there must be a healthy population present by the time its deep into flower.
Plus the fact they are cheap and more then likely it’s a container of soil that once harvested it will be threw out or remended before it’s reused anyway.
So in my opinion, I see no reason to use them other then 3-4 times through out the entire grow and definitely no reason to twards the end say last 3 weeks or so.
But just my opinion and works for me.
My main source of microbes in soil comes from this product. I mix a good bit in the soil a week before I plant a seedling in, then i top dress a couple of times threw out by flower time I start feeding synthetics and stop worrying about microbes.
In my coco-coir plants I use no microbes at all but rather h2o2.
This works well for me but others will have different approaches that will also work.
Some of my soil and coco-coir plants for reference.