I tested out all the pioneer shields (aka ancestral shields). The block rate below is that of a veteran fighter (agility 6) using Lv.25 Back Guard. Appearance indicates which normal shield (available from NPC) a pioneer shield looks like when equipped. The block rates of other stances (Lv.25) can be calculated accordingly: High Guard (+22), The Defender (+26), and Stave Guard (-7).
The Lv.80 pioneer shield is only slightly worse off than Lv.84 Elite Royal Guardian (DEF 22, Block 41). Of course, being a pioneer item, pioneer shields do not end up with "blanks" when using enchantment chips. But the bad thing is that you cannot get DR+2 option on the shield.
Edit: Thanks to DeLaFeuillade of Orpesia for additional information on Lv.100 pioneer shield!