Small accelerations over long periods are great! But when you're using electric propulsion, how small starts to become an issue. As far as I'm aware ion thrusters have TWRs in the 1/1000 range - NERVA's were in the ~1 range. This means you're taking a 3000 second burn and replacing it with a 3,000,000 second burn - add in efficiency losses and things start getting interesting. (assuming a constant mass fraction devoted to engines, and that the electric propulsion TWR includes power generation and cooling)
Hydrogen arcjets are going in between 1300-2000 seconds ISP, and are 30%-40% efficient, which is huge by electric standards of electric propulsors, and are can be done with tens of newtons thrust, with 100+ newton per engine deemed possible.
And you can get 1300-1500s ISP from a liquid core NTR, too - or 3-5000s from a gas core NTR. Sure, no one's ever built a liquid core NTR, but there have been designs made.
And needless to say, "tens of newtons" is not the projected thrust of a liquid core NTR. More like a few hundred thousand.
Scalability, and scale matter too. I believe that solar electric thrust to weight ratio is very favourable with modern solar cells, and scalable.
Arcjets can be really tiny, and you can have hundreds of them. Given that you will also have to get huge amount of electricity for crew needs, you will have to pack solar cells anyway. A bimodal NTR will be even heavier, and require even bigger vehicle to legitimise its use.
Only minimum scale really matters - past that you can just cluster engines to get the desired acceleration. Minimum scale for any sort of nuclear thermal rocket is below what you'd want for a manned interplanetary mission, and is thus not relevant.
Solar power plus ion engines is in the 1/2000 TWR range as far as I'm aware. That means millimeters per second squared acceleration of your total craft at best, and means you basically don't save any time over a standard minimum-delta-v Hohmann transfer - 0.001 m/s^2 continuous acceleration gets you 2 AU in ~400 days. It could also do 66,717,283km - the Earth-Mars distance at time of writing - in ~189 days. A Hohmann transfer from Earth to Mars is 259 days. And, of course, the above numbers don't take into consideration matching velocities or escaping Earth's gravity well in the first place. [1] does a good job of describing why the power supply is the primary limiting factor here.
Liquid-core NTRs [0] aren't bimodal, and I'm sort of confused what you'd mean by bimodal here in the first place.
6.4kg 50N 30%-40% efficient hydrogen arcjet, and 1kg/kw solar panels will probably scale up until a 1.5-2kn, which is really a lot of for a relatively efficient, 1000s+ ISP engine made using existing material science.
Current hall-effect engines can produce thrust for 50,000 hrs (~5.8yrs) before needing refurbishment. This is enough for a few trips to Mars.
Even if they cut a week or two off the travel time, they might be worth it on craft with humans as an extra week or two of life support is fairly heavy (~2kg/day/person).
Ion thrusters = technical problem. Thermal rocket = fundamentally limited by rocket equation.
With thermal rocket you need huge amounts of reaction mass because it is expelled at slow speeds (it gives little push relative to its mass) and then you need more reaction mass to push that reaction mass and so on. This hugely limits what you can do.
Ion thrusters are largely technical problem of erosion. Current designs have trouble withstanding continuous load because ions hit electrodes and erode them. But there is no physical limitations. Superconducting electromagnets, maybe something else. Somebody hopefully gets a good idea and gets reasonable thrust from ion engine.
Ion engines are still limited by power density - higher ISPs take quadratically more power input at reasonable ISPs. (As in, at non-relativistic velocities) Thermal rockets have a lower upper bound, but come with higher thrusts. Everything is tradeoffs, in the end.
And why on earth would the kind of newtonial engine mean that you wouldn't be limited by the rocket equation? The only escape that is to have reaction mass outside of your reference frame somehow. (picking up fuel from interstellar space, having thrust beamed to you via laser, some form of reactionless drive...)
No body are talking about plasma rockets (VASIMIR). Elctric rockets that have variable ISP on demand (low-thrust, high–specific impulse exhaust or relatively high-thrust, low–specific impulse exhaust). Also, VASIMR does not use electrodes so not erosion problem.
