"Harbor Freight Palm Nailers"
HF's motto should be, " Where the customer is quality control."
I've never used their palm nailer, I have a $20 brad and staple nailer that I have used and abused for for over 10 years, it gets plenty of use and I have never had a problem with it. I have a $60 multi-angle framing gun almost as old, no problems with it. The first Multi-function oscillating tool I got from them just stopped working after a haqlf hour, the replacement has been cutting every thing from cast iron to vinyl tile for 5 years.
With their no question 90 day guarantee, 30 day period to buy extended warranty, sales and coupons , it can be worthwhile to risk the small amount of cash. Especially for a casual user. If the store is not so far away to make the return trip a hassle. When you get the tool use the heck out of it, even to point of abuse, try to wear it out before the warranty wears out. If it lasts that long it will probably last. If within 30 days its still seems OK the extended warranty may be worthwhile, but with sales and coupons it may be cheaper to buy a new one.
Of course for a casual user of an unfamiliar tool, it may be difficult to determine if the tool is not working exactly right or if the user isn't using it exactly right. Usually a HF tool works fine or doesn't work at all. Usually a HF is as good or better than low end tools from HD or big boxes, at much lower price. Often they are same tool.
With a low end palm nailer from any where, you are liable to get as much pounding on your hand as on the nail. For a DIYer on a smaller job, that might be bearable. And you could rig up some extra hand padding.
As to shear strength of nails vs screws; Just go to a bigger size screw. A 16D nail gun and a #8 screw both have a shear strength of around 90 lb. http://www.builderonline.com/construction/dear-builders-engineer-nails-or-screws.aspx
When sistering I use some screws to pull them together anyway. I usually use some lag screws or bolts, a wrench won't bugger the head like a screw driver can, and you can get a wrench or ratchet into tighter space.
HF sells angle drill and angle adapter for drill, so you can drill pilot for lag screw or all the way for a bolt. ( BTW I am leary of HF battery tools, in low end tools its the batteries, it seems they don't hold as much charge and wear out faster.
That is some "handiwork" indeed, they've turned those 2X 12(?) into 2X6's. The piece you can wiggle is doing nothing but holding up the pipe. I dunno what the background one is doing, mebbe holding itself and the light bulb up. That far pipe hole is awful close to the edge. You may have to chisel out some brick to get some end support for new joists. Or put some columns under ends. How much of a chore to disconnect pipes and bore the properly sized sisters properly?