So for the new building system, I originally thought the reduced number of mats would be a bad thing and might compromise some of my building designs due to the absence of rcon. This wound up not being the case for the final product (i.e. the A20 version of structures I built in A19 are just as hardened), so I happily digress my concerns there.
The new building process, however, is something I have more feedback on. I'll front-load by saying I've been hardcore base building since A11 and have a strong grasp of designing things in the game and managing the material logistics for my group when we get rolling:
First the good. Overall, the construction process feels faster. Being able to throw down finished concrete blocks without waiting for wet crete to dry or having to wait a second round for rcon is a big plus. The absence of rcon also cuts the material cost of advanced builds in half, and the many MANY new shapes enhance the complexity and quality on structures - and made it possible for me to do stuff I couldn't in A19 or before. Kudos all round here.
Now the bad. While having all the new shapes available as frames is nice, the process of upgrading framed layouts into finished structures for advanced builds (concrete, steel) is now harder / more time consuming. Previously, it was possible to do structural mockups for a hardened base with rebar, then just nailgun everything. Now, I find I have to go back and remove the framing, then place the final crete or steel block or fully upgrade the frame - which requires additional materials and IMO creates waste if your goal is to initially build with higher-tier mats. Yes, the XP gain from doing full-chain upgrades somewhat balances things if you go from frames up to crete or steel, but the layout phase of building just feels very different and harder, even if it also has the potential to be faster.
As my team's builder, I used to get most of my XP from building our base out of rebar and upgrading it to rcon. Now getting comparable XP requires throwing a bunch of extra wood and cobblestone at framed mockups and then going up to concrete. Admittedly it doesn't sound like a big change, but the base I'm used to building for a group of 6-8 players would typically involve 3 or 4 thousand blocks, all made out of rcon. Managing logistics isn't an issue - it's still 2 materials, just wood and stone instead of wood and iron. But managing 3 layers of upgrades instead of 1 is more difficult.
Also, accidentally placing a finished block can now be an expensive error if it's concrete or steel. One more consequence of removing rebar.
Last, as someone who relies heavily on precise dimensions for my builds, I've found it tougher to perform measurements without painting or otherwise adding marker frames, whereas before just counting the number of rebar / rcon blocks made things simple.
Don't get me wrong. I'm not railing against the decision to evolve the building system. Treat the above as a knowledge exchange. But I think one way to address everything I'm pointing out would be to bring rebar back in some form, even if only with the A19 system of shapes available for it, and just have it be upgradeable to finished concrete. That would resolve the time and resource sinks for removing/replacing frames or fully upgrading them, and make it less likely for builders to have to destroy 5-10k HP blocks placed in error.
Sorry for the long post. Nothing but praise from me otherwise on the changes to materials, shapes, and overall construction.
TLDR: I love the new build system but I miss rebar!
Happy birthday, little sound bug, happy birthday to youuuuu. 
