South African here and UNISA alumni. I've completed a Bachelors degree with UNISA and doing my second degree with them currently.
UNISA is not really an online university. Rather an open distance university. What's great about UNISA is the ability to study while you work. I work full time and study. The cost of UNISA compared to other universities (in SA) is also very competitive. If you're going to majoring in Physics and Chemistry you will have to do some practical work at a regional facility. See the UNISA website for the one closest to you. If you're based in or near Johannesburg, check out the Science Campus - it's really amazing.
Most UNISA students also aren't aware that UNISA boasts the largest Academic library on the continent of Africa, with over 2.7 million items. No other University in SA even comes close to this. I'll stop before I turn this reply into an infomercial.
In the past UNISA did have some administrative challenges, or so I've heard. I'm either really lucky or people exaggerate issues perceived - in my 9 years as an active student I've never experienced any issues.
As a last note there's quite a few student groups on platforms like Ryver (https://ryver.com) that will help a lot if you're unable to attend discussion classes.