Your suspicions are correct. Coles mobile data is crap. It's the worst I have ever used.
I bought the 12mo 60gb $100 deal.
I have been using it for over a month now, not a power user and no streaming. Most times, and all of today, mobile data simply does not work. Sites like Ozbargain, Reddit and general Google searches often time out, sometimes load in 30 seconds, sometimes it's minutes.
Phone calls and SMS seem ok.
Previously used Kogan and Catch and they were ok. Coles isn't just slow, slow would be fine, it just doesn't work. / end rant
Tldr - Coles mobile data is crap, avoid.
Edit, using in Syd middle NW and N.