Yes, TPG might have the worse coverage, but my previous experience with them about 1 yr ago was definitely constant improving, at least for most of the place i went to, there is decent connectivity. What Vivifi throws me off completely is the "deprioritisation" to the extend of 0 data throughput. Imaging doing a paynow transaction and you are stuck there 2-3minutes for the data while the queue form behind you. I don mind slow, issue is it went complete no data.
In fact i don't remember having such issue with Zero1 which is also under Singtel infrastructure, at least on a very short duration with them.