Some people have criticized BIP9's blocktime based thresholds arguing they are confusing (the first retarget after threshold). It is also vulnerable to miners fiddling with timestamps in a way that could prevent or delay activation - for example by only advancing the block timestamp by 1 second you would never meet the threshold (although this would come a the penalty of hiking the difficulty dramatically).
On the other hand, the exact date of a height based thresholds is hard to predict a long time in advance due to difficulty fluctuations. However, there is certainty at a given block height and it's easy to monitor.
If there is sufficient interest, I would be happy to amend BIP8 to be height based. I originally omitted height based thresholds in the interests of simplicity of review - but now that the proposal has been widely reviewed it would be a trivial amendment.