As a campground, Elk Falls is similar to other BC prov. parks. Good sites, lots of separation, modest amount of walking (e.g. Beaver Pond trail). It's a good stopping point when headed to other places, and for restocking in Campbell River. Miracle Beach is 100% reservable, Elk Falls is less the half. In September that difference is less important.
I don't think these access warnings apply to the campground part of the park. The campground is on a tributary of the Campbell River. The Falls are further up river, just below a dam. This a day use area, separate from the campground.
Elk Falls trail mapThis trail map doesn't show the campground
Of the area parks, I liked Ralph River best, but that's with a small RV. It has older forest and access to hikes at the south end of the lake. Not good for swimming, though.