As long as you use full strength it definitely is. The way spinosad works is it's derived from a bacteria, it coats your leaves so everytime those bastards bite into a leaf they drop dead.
It also works on thrips! Sometimes though, you'll give your ladies a spray and you'll notice a few mites walking around, as long as you've covered your plant when you sprayed have no worries they'll end up dying anyway but to be on the safe side spray the first time, then spray again 3 days later and spray one final time 3 days after that.
As far as spraying in flower goes, I've had an incident where mites didn't show up until week 3 of flower and they infested FAST! This was my fault as I didn't take any preventative steps to keep them from ever showing up. I ended up spray week 3, didn't see them again til about week 7 or 8 then I was spraying on and off til the day before harvest. It wont damage your buds, just make sure your room is well vented.
I wouldn't recommend neem oil though unless you're going to be VERY careful as thc is not water soluble but it is an oil and gets easily washed off with other oils!!
seriously tho, if you have mites the best way to get rid of them is this stuff ? cud u use during flowering and could neem be used during flowering?