[Logtec]

I do enough grab bars,
it’s always best to hit a stud on both ends of the grab bar, “drive grab bars” have a swiveling flange to help with that.
But you can’t always hit the stud, depending on the length of the bar or the angle it’s to be installed at.
I use a Bosch stud finder with deep scan which hits 90% of the time in homes with wood/steel studs.
On the holes that don’t/can’t hit a stud I use these cobra wall plugs(see below, heavy duty, once they bite in- they really hold, I’ve been using them for years never heard a complaint)

Note: I avoid apartment buildings and condos for grab bars- it’s almost impossible to find/locate the thin metal channel used to secure the wall board.

 

[Logtec]

In my Apt. when I asked for grab bars for my tub/shower they installed them Vertical outside both ends just so they knew
that were going into studs, even though they still only managed to get two screws into the studs

We usually install one vertically on the outter edge to help getting in and out of the tub/ shower and one horizontally or on an angle (45 deg-ish) inside the surround to aid in bathing from a sitting(chair/bench) or standing depending on the clients needs.