I am pretty much convinced that the lower frequency limit in VNA mode is due to the directional coupler design. As I mentioned before, I am somewhat amazed that it actually works down to that frequency.
I don’t foresee any change other than a redesign, and probably significantly increased price to “fix” this.
If you want to roll your own directional coupler for lower frequencies, you could use it in SA mode and use the tracking generator. It isn’t a VNA when used like this, but for checking antenna SWR and filter frequency response it should be good enough for most ham purposes.
I bought one. It only arrived last week, so I haven’t had time to really play, but I did use the VNA to look at how my old 2m/70cm antenna is working (well, apparently). SWR and Smith chart views were good.
Also switched to SA with the antenna connected as input, restricted to 144 to 148 MHz and watched the signals appearing and disappearing. Better display of this than I get on my radio ... but so I would expect.
I was a little dissapointed at the low end coverage, but if you want a combined sa/VNA and all the other options going up to 1.5GHz, it is going to cost a lot more.
My biggest gripe is the cost of the options, and just how much of the functionality is an option.