The Rainmaker is a very elaborate device: 16 taps, for starters. Each tap then has its own filter and also a granular pitch shifter, and then things start getting VERY crazy. Plus, it has a comb resonator with from 2 to 64 taps, which actually gives you one more voice in that you can use the comb resonator for Karplus-Strong "plucked string" modelling...or just use it as a tunable digital resonator which then can provide something like the Rings (which would then mean you could remove that too...more room!). Compared to the Strymon...well, it blows that out of the water, pretty much. The DLD, however, is not so much a delay as it is a dual looper...a delay with a buffer that loops, times two. The Rainmaker doesn't exactly do that, but it has a buffer "freeze" function, like the old one-shot delay/samplers of the early-mid 1980s, with control over buffer playback direction. If I were to sort of look at this simply, though...the DLD is a looper, the Magneto is a delay, and the Rainmaker is a time-domain instrument. More elaborate, ultimately, for only $40 more than the Strymon.
The reason for using the Mutant Brain lies in the fact that you have the same amount of CV/gate pairs as the Yarns...AND eight more gate outs, which means you can assign (via MIDI Sysex) those to other gates, clocks, etc whatever, as long as its a 0-+5 gate/trigger output. This then allows you to run all four voices and the clock...and more besides...as opposed to the Yarns, where you only have that set of four paired CV/gates. The drawback is in not having the internal control the Yarns has, but only up to a point. Consider: if you can alter the MB with sysex, and reconfigure it on the fly, you can load sysex messages from the Digitakt that can alter the MB's gate/trigger allocations/parameters as needed, or from a computer connected through the Digitakt. In short, it's actually got a feature set that lends itself more to a clock-driven system such as this.