Installation: Solid iron balusters are drilled at the top and bottom, secure with epoxy and wedged at the top and bottom. If placed into solid wood cap (and not laminate flooring) you cannot kick them out or bend them.
Hollow balusters have a lag bolt that is drilled into the cap, with an inch threaded top to secure the baluster too. Not sure how the top is secured. My hope is they are shimmed at least. I have included the instruction they give out with the hollow balusters below for your review.
Hollow balusters are not secure - cheaper yes,
but like anything else we buy - You get what you pay for. You want cheap - buy cheap.
Rails from China - the rails are Finger Joined. Two things wrong with that - weaker and if it gets wet it will come apart.
Where as the Rails made in the US by a reputable Stair parts house are "stacked" as you can see below. Much stronger and the seams do not show when stained.