Never used or heard of a spray for boots worth a damn. Have used mink oil, snowseal, Danner boot dressing, and other ointments, greases, waxes that I can't remember brand names of with various levels of effectiveness other the years. One thing I've learned is that there isn't a single product that works for all leathers.
I think that your leading statement is the key!
In today's world, we have a lot of different products, that we're dealing with!
Like you said "Single Product for ALL Leather's"!
There's so many products out there that we're trying to deal with, and keep,
Sealed, and Maintained.
Who knows what we're dealing with half the time?
Back to your Original statement, as I said Earlier, that I learned for Real True Leather,
that Bear Grease is all I use!
My old buddy used to say "Something Slippery is Better than Nothing at All"!
With all this fake Leather, and other products that are imitation Leather,
and Look alike products.
I use whatever works for that stuff in the Family, but Real Leather!
Bear Grease is still readily available!