If you want a sub to carry unlimited range low flying nuclear powered cruise missiles of unlimited flight range... and there is no reason to think they do at this stage, but if they did then a large platform like the Akula would be better suited in terms of the numbers of missiles it could carry and also the ability of the sub to endure very long deployments all around the planet, which is critical because you want to sail enormous distances for very long periods with such a vessel because away from the chokepoints in the north like the GIUK gap etc there wont be a lot of sea bed sensors available to track these subs so essentially the US or one of her allies would need to actually follow the sub around to keep tabs on it... and you couldn't just send one ship... you would need to send a group which just adds to the costs.
The Russian sub in question doesn't need to be super stealthy, or particularly fast... for all the west knows it might have self defence drones that intercept incoming torpedoes and attack the platforms that launch torpedoes at the sub... but really so far it has been used for testing Bulava SLBMs, and having a one off sub that has no other duties than testing is useful because it means it is always available and never gets called away to service and can do mundane long term testing.
It means Yasens and Boreis and other new subs don't need to be called off duty to test things... sensors, weapons, equipment, etc.
The new electric drive concept means new ships and subs will be electric drive vessels, so no big engine attached to enormous shafts on the end of which are huge propellers.
The new electric drive concept means the wheels or propellers contain the electric motor and rather than attach a big diesel or nuclear propulsion system on the end of that, you can have a couple of smaller "engines" that generate electricity.
I remember as a kid when batteries were not cheap and were not very good in terms of capacity and were heavy and large for their performance, so my bike had a dynamo generator... which is just an electric motor that was driven by the wheel of the bike that was connected to the light on the front.
Advantages included it was cheap and reliable, but problems include when you stopped the light went off, and if you went down a steep hill the light got very very bright but if you were pedalling up a hill it got rather dim.
The modern solution would be a rechargeable battery and an electronic controller. Instead of connecting the dynamo directly to the lights you connect the battery to the electronic controller which checked the state of the battery and its level of charge and its capacity to take charge, and then you would connect the dynamo and the headlight with a switch so you could turn it off during the day and on at night or low light conditions.
As you pedal around with the light off the dynamo will simply charge the battery, whereas at night or in dark conditions you turn on the light which will remain bright whether you are pedalling or not.
The electronic controller will tell you if there is a problem with the battery and manage charging it so it does not over charge or overheat.
Modern LED lights would reduce the demand for power from the batteries and make the whole system very efficient.
For all the same reasons disconnecting the power plant from the propulsion in Ships and Subs is also a good idea.
Normally with a sub hunter like the Udaloy class ships you have low speed diesel engines for normal operations up to about 18 knots... usually you have two for redundancy if you have problems with one or battle damage to one you always have a back up, but the ship design might need 80,000hp to go 32 knots, well a 40,000hp diesel is huge and heavy and expensive, so having two 20,000hp diesel engines for normal operations, plus two gas turbine engines for high speed operations chasing down a sub that add another 40,000hp with the two of them means most of the time you run two or even just one diesel engine because it is fuel efficient and generates the power and electricity you need to run everything and operate at low speed to move around the place... when you need to go faster you can start up the other diesel engine and the two gas turbines if you need maximum power.
A new ship or sub will likely work the same except with nuclear power plants... so you would never fit an 80,000hp NPP to a sub, actually having four of 20,000hp makes more sense because you can pick which powerplants to run and you might only ever run on all four NPPs for very short periods... which means the NPPs you use the most often to start with can be used as the very occasional high speed burst engine later on in the life of the ship and the NPPs that have been mostly rested can take on the work of normal operations.
It also means if a NPP fails you are not dead in the water.
Having electrical propulsion means you can put the NPPs where you want, and in fact they could be located in places where you take off a few outer panels and slide the NPP out like a battery... you don't refuel it... you take it out and replace it... and the old NPP could perhaps be retired to generate power for a small town or island, or refuelled and put into a new ship or kept as a replacement battery for the ship or sub it came from.
Lots of things to work out and learn about and lots of new technology to be developed including better batteries and better capacitors and electrical current and storage management systems and equipment.