I am soon to head to my daughters home to help install a floating wood floor and replacement stair treads. The treads will be prefinished Briz cherry. Not sure if the treads they ordered are full thickness or 1/2" that just go over existing tread. The stairs are currently carpeted so I'm sure the treads now are either plywood or pine.
My question is- having never worked with briz cherry how hard is it? Can I shoot a 15 or 16 ga finish nail in it? I plan to pull up the current tread lay down lots of construction adhesive(liquid nails), then shoot a few finish nails to help till the glue sets. Or would it be better to install screws then plug over the screw heads, which might be hard due to the tread being already prefinished.
Any suggestions or hints? I am unable to have access under the stairs as drywall has been installed and finished under them.

Never heard of Biz cherry. Probably just a name for a finish or color by tread manufacturer. They can some times take a lot of liberty with names. There are several species of cherry from many countries. Also, importers have been known to import some wood from South America and elsewhere and call it a certain species with a name in front of it as a qualifier.

They do this claiming it looks like, or has some resemblance to a certain species. I can not imagine you not being able to shoot a finish nail through it. I would think you would have to wait until you see what you are working with before deciding just how you are going to do this task. Hopefully there will be some instructions with the material. If not, I suggest calling the dealer but don't expect too much. Chances are that they are just retailers.
Chances are that screwing and plugging would be best for installation but you have the problem of staining and finishing. A difficult if not impossible task if the treads are prefinished.

For that reason you will probably end up gluing and nailing. Regular wood glue usually requires clamping to get a good bond. I would consider a construction adhesive and as few nails as possible. The construction adhesive will take considerable time, as much a 6 months, to completely cure. But it has much better gap filling qualities, the bond is quite good after it has cured and does not require clamping.

Good luck

A review of the Janka scale of hardness puts Jatoba hardness the same a mesquite. I am not familiar with either. Here is a link to a very good woodworking web site
There is an individual, Scotman (in Texas, I believe) that works with mesquite. Might want to consult with him.

Of course you could always do it the old fashion way, drill a pilot hole and then use a hammer and a nail.

Cut the head off a finish nail, chuck it in your drill to drill the perfect size hole. Nail through a thin piece of wood such a 1/4" luan to avoid accidentally denting the wood. Set nail head beneath the surface with nail set. Fill nail hole with wax crayon.