As with all things in life, the answer is “it depends.”
It depends on many things – how you play it, against whom you play it, which lines you play. And also there is the distinction of “simple” and “simplified.”
I made the transition from 1 e4 to 1 d4 back in 2008 when I realised that I won’t be becoming a GM soon. My rationale was since I wasn’t getting the title then I may as well study and learn something new. Creating a repertoire is never a simple task, but I knew what I wanted. I wanted safety so I immediately started with “everything g3.” With the benefit of hindsight, not a bad approach. Later on I expanded on it, but the love for the fianchetto has remained ever since.
Chessable’s author IM Christof Sielecki followed up on his Keep It Simple: 1 e4 with Keep It Simple: 1 d4. Not surprisingly, he also advocates the fianchetto against pretty much everything.
After watching the introductory video I got a general idea of all the lines he is suggesting. I very much liked his sincerity where often he would say that Black is fine in certain lines, admitting that there is no advantage in the variations he’s suggesting. And this is true, as in modern theory it is impossible to claim an advantage in any sound opening.
Christof’s “simplicity” is shown in the suggested move-order for the repertoire: 1 d4, 2 Nf3, 3 g3, 4 Bg2 and 5 0-0 against pretty much everything. But then things get complicated.
The repertoire does not shy away from entering mainstream theory and mainstream theory is anything but simple. The author recognises the danger of suggesting off-beat lines just in order to avoid theory – this is a dead-end street in the long-term – so he enters the main lines when that is the correct way to go. Good examples for this are the transpositions to the Catalan, the Fianchetto Grunfeld, the Tarrasch Defence and the Queen’s Indian.
I checked several lines that were interesting to me. I was curious to see how the repertoire compares to my notes.
In the Tarrasch Defence the proposed line is to play the main line with g3 without Nc3. I discovered that Sielecki improves on the proposed line by Aagaard and I couldn’t find anything wrong with it.
In the Reversed Grunfeld lines (1 d4 Nf6 2 Nf3 d5 3 g3 c5) he digs deep, following a forced line from many games and then considers 4 candidate moves for Black on move 14, with White having a pull in all of them.
In the Catalan you cannot avoid theory and the repertoire doesn’t intend to. The good aspect of the proposed move-order is that it avoids all the Catalan lines with early 4…dc so Black’s choices are limited to the Closed System without the subtlety of …Bb4-e7 or the Open System with 6…dc.
In the Open System the very recently played game Grischuk-Nakamura from the FIDE Grand Prix posed serious problems to Black in the line 7 Qc2 b5. Sielecki couldn’t have known of this game when he worked on the repertoire and his analysis follows an earlier game, Gelfand-Ponomariov from 2011, but stops short after 16 Nc4, limiting himself with a verbal explanation when theory goes on and Black is probably fine later on. As with all theory, it never stops evolving, and the improvement by Grischuk (16 Nd4!), who in turn was following an analysis by Avrukh, will serve well the diligent student.
In the line 7 Qc2 a6 the suggested line is 8 a4. This leads to microscopic (if any) advantages for White, but it is extremely solid so it fits well with the whole idea of the repertoire.
I was somewhat surprised that against the Grunfeld with …c6 the recommended line is 7 b3, allowing the sharp equalising try 7…dc 8 bc c5 and after 9 Bb2 Qb6 10 Qc1 (instead of the more popular 10 Qb3). This is a very concrete line requiring good memorisation, somewhat at odds with the generally slower pace of many other lines.
Against the KID the repertoire doesn’t enter the main lines but suggests 6 b3 instead. Apart from the move-order issues (as Black can play …c6 instead of …d6, toying with an idea of a transposition to a Grunfeld) this is a very practical approach – not many KID players like the sight of 6 b3 and the lines are not bad at all for White either!
The only lines where White doesn’t fianchetto his bishop are the ones where Black plays an early …c5, where the suggested plans are based on playing d5 and follow mainstream theory which guarantees White an advantage.
As usual, Christof is very thorough and I couldn’t really find a weak spot in the repertoire. Curiously enough, he often says that he used LeelaZero to suggest interesting ideas.
After going over the repertoire I cannot say that it is “simple” at all. It is simplified to a certain extent, but it requires a lot of study in order to absorb the whole material. If this doesn’t bother you (and it shouldn’t!) you have a very solid and high-quality material to rely on that will most likely serve you for many years to come.