You make a great point! 
I don’t think any valuation is inherently perfect, all you need is to consider two estate agents with different listing photos, a desperate seller or keen buyer to consider that there’s things outside of any valuation model that will impact an agreed sale price or indeed the valuation that a surveyor sent by a lender gives.
In general, we’re favouring financial empowerment by offering folks more information to help them understand their situation better, especially if it has been a while since they bought their home and they don’t have a benchmark (or Halifax) to give them an estimated value - and we make it clear that you can easily edit which Halifax don’t on their “equity view”.
In my own situation, my Halifax valuation was pretty solid compared to my own understanding but Metro was considerably off the mark even when I had moved onto them recently 
For what’s it worth, I do think >300 segments give us a strong foundation and there’s varying opinions of which indices are most relevant in a given scenario:
- Lender indices (by definition) are only mortgaged transactions and have biases towards their lending policy & residential mortgages and are seasonally-adjusted
- ONS covers all transactions and don’t apply any seasonal adjustments
I wouldn’t normally defer to a Forbes article but I think they do a reasonable of summarising the approaches in case you’re interested - it gave me some good evening reading! 