The Mann et al. conclusion that recent high global mean temperatures are exceptional has been verified by many studies.
Reconstructions show a wide range of estimates for the strength of the negative temperature anomaly from 1700 to 1900 ("Little Ice Age").
The statistical methods used have been verified by studies using pseudo-proxies from climate model simulations, but there are still gaps in our knowlegde about the appropriate noise structure (power spectra, coherence, correlation scales, etc.).
Methods which explicitly account for properties of different proxies on different time scales may improve the skill of reconstructions.
Different forms of regression can lead to significantly different results during the highly anomalous period 1700 to 1900, but have little impact on the period around 1000 (Song dynasties/"Medieval Warm Period").