Used to be a nurse's assistant.
It very much depends on the chair. The typical push chair, like hospitals have available is a hell no.
Manual chairs that you'll typically see owned by individuals, can do okay in a well maintained grass area. But the front wheels are prone to digging in, and it is harder to roll when you're in the chair than something like brick or concrete, obviously. It's on par with gravel, but they're difficult in different ways.
Power chairs, the difficulty matters less, but you're dealing with extra weight, so that brings worse issues with sinking in if the soil is loose or over wet.
Level surfaces are always better than uneven surfaces, obviously, and soil is going to become uneven over time if there's regular traffic over it. So, if you're providing an accessible route into a business, you'd want something more durable. If it's a private residence and you're setting up for visitors, it would do fine overall, but expect bigger maintenance costs and effort if they're frequent visitors.
If the chair is for someone living there, grass and soil would be at the bottom of the list, with only bare soil being worse imo. You'll chew up the grass and have ruts inside of a few months at best, if the person is out frequently.
My pick for an access route is obviously something like concrete or asphalt, with a well laid stone path being a good choice. Reason being, you want as even a surface as possible for regular use. I've seen good paths done with pavers, bricks, natural stone, etc. The only real issue is that long term use is going to be dependent on what's under the stone. If it's well packed, and done right, you'll get decades of great evenness for sure. But if the job is half-assed, the pavers can shift and become uneven in a few years time.
The quick and dirty option is gravel laid down thin, then refreshing as needed. You get a thick layer of gravel, like for a driveway, wheelchairs suck to use. But just enough to lay on top of soil and reduce and sinking and rutting, while improving traction, you can get a fairly stable surface. But, you'll have to maintain it way more often. Any time it rains, it's going to soften the soil, and the next few uses shift it, possibly creating ruts. So, expect that and keep some gravel on hand, and invest in a tamper.
But, all of that still depends on the chair and the driver of the chair.