By the time of the BRANDENBURG-class, Germany was a newcomer to the small splendid battleship designing nations. As a relatively young nation with naval ambitions it was clear that something unusual would appear from the design boards.
For a number of reasons the BRANDENBURGs are innovative but the armament certainly is not a big factor in those considerations as outlined below.
[+]1895 there was no armour piercing shell aviable to defeat face hardened armour in an intact condition, provided with a filler insensible enough to survive heavy impact shock and a fuze reliable enough to allow in depth penetration before exploding. Technology level in this time included nose hardened APshot or-shell (the latter with black powder filler).
[+]1895 there was no centralised firecontroll aviable, no continous aim, no telescopic sights, no rangetaking at all, except MK I eyeball. This precludes very long range encounters. All period encounters until the russo-japanese war were point blanc range engagements from the gunnery perspective. Gunlayers were obligated to aim in the roll to either hit the upper works or the waterline and fired on gun-range, not target-range.
[+] In the 1890´s Compound and Nickel steel armour gave way to the very first face hardened armour, either Harvey or KC, which was able to reliably shatter all period shells (soft capped APC didn´t appeared before 1902 and capped, delay action APC weren´t introduced before 1912). This casts even holing of armour in severe doubt.
These two limitations and the adotpion of face hardened armour drastically reduce the tactical usefulness of major calibre guns in this period at normal figting range. Their exploitation was furtherly hindered by smokey burning propellent, which made necessary to clean the barrels between firings in intervals and in doing so hindered firing rate of all guns. Major calibre guns were housed in Coles/Ericcson turrets or open mounts with very few true turrets existing at the time of the BRANDENBURG.
On the other hand, this timeframe saw the introduction of Quick firing Batteries, smaller calibres 6in down to 3in, able to achieve very high firing rates. Instead of the one shot in two minutes these guns could fire twice a minute and later with smokeless powder and better handling practices these firing times furthely reduced to one shot in 40 sec. for major calibre guns and 4-6 rounds per minute for 6in QF. It also saw the introduction of high capacity shells in most navies, which lagged a penetration ability but on the other hand were able to wreck any unarmoured structures around.
Thus, in this timeframe, the BRANDENBURG may seem to represent a forward looking battleship design, but one not really fitting the periods greatest threads.
This danger was not major calibre fire but rapid fire of medium batteries with HE-shell, degrading the ship´s superstructure, causing a large toll of deaths and injured aboard. It was firing practice to close the range as much as feasable (1,500 to 3,000 yard, rarely more), continue on parallel courses and fire HE shells until the targeted ship is disabled, abandoned, set ablazed with fires out of controll or driven ashore. Then with a few well aimed AP-shots on the waterline with major calibre AP additional flooding was caused on one side of the ship to hasten it´s sinking. This combat doctrine can be traced down in the sino-japanese naval battles, the hispano-american naval battles and even the russo-japanese ones, even though the latter also saw the attempt to create some long range actions.
SMS BRANDENBURG and SMS WÖRTH both had relatively thick compound armour at the waterline but no armour as citadell protection, thus these ships were very sensible to HE fire even from the smallest calibres. They may have had face hardened armour on the turrets of the Gruson type similar to the armour employed in german land fortres turrets. While the latter two ships of this class were the first ships to have Krupp cementated armour for the belt and barbette, still no upper side belt was provided for them. The main guns were housed in innovative turrets but required end-on-loading (all-round loading was still in the future for all navies) and had a relatively long firing cycle. I don´t think the difference in calibre length has anything to do with different ballistics for period engagements and spotting or salvo rules were simply non-existent in this period.
The Majestics on the other hand had a thinner main belt, but equally made with face hardened armour (HARVEY), which would be suitable to defeat period AP shells from ca. 3000 yard figthing range and additionally they had a 6in upper side belt which would be effective in stopping smaller calibre HE destroying the upper hull. They didn´t had true turrets but fielded a useful medium battery firing HE-shell, which the BRANDENBURG lacked.
The RMA learned from this experience and the succeeding KASIER FRIEDRICH III class was basically the opposite of the BRANDENBURG. After analysing period naval actions, QF fire was considered of utmost priority. The KAISER class fielded 18 (!) 5.91in QF guns firing a proper HE shell and even the main guns were modified to adopt the QF-principle (the largest guns to use QF principles at this time).