I installed grab bars in a tub/shower in our house with plastic anchors sized for the stainless screws that came with the grab bars and these are holding perfectly after years of service. I then used these anchors to install bars in a walk-in shower with 4 or 5 inch sill. I use a stud finder to find studs but our studs (24 in on center) are not always where I want to place a bar. (We remodeled our two bathrooms years ago when we didn't realize we would ever need grab bars and so did not put horizontal blocking in place.)
When I started installing bars I intended to secure into studs or use Wingits, but I misused the stud finder on the first bar I was installing--the drill went through the ceramic tile, through 1/2 in cement board, through 1/2 in drywall into . . . emply space. No stud! #$@&##
This was an 18 inch vertical grab bar at the outer edge of the control wall of a tub/shower (so high step over!). I decided to go against recommendations and use the holes to see if the anchors would hold. This was the shower I use so I knew I could evaluate whether it was holding or not. When I decided it would work I used the same technique on the 2nd bath in our house and a bathroom in another house with similiar wall construction.
All the shower/tub grab bars I have installed with plastic anchors are completely solid after 7 or 8 years. I apply full force with my right hand/arm getting into the tub (left hand stepping out. I am 200 lb and 78 years old. I don't want to slip getting into the tub.
I have a horizontal 36 in bar on the long wall in my tub/shower also secured with the same plastic anchors. It is also solid.