The biggest hurdle is that cancer cells move around within your body, alongside multiplying (even if you opt for chemotherapy: there's a probability that'll resist to the medicine or cause more harm than good). Also, Cancer cells aren't foreign invaders like the viruses, bacteria, etc that make humans sick, rather they are the person's own native cells that have "gone rogue" through mutation, making the process on finding a cure complicated.
If you really want to create a permanent universal cure: you will need to study and experiment literally every variation of cancer (it differs by individual too, as the treatment may work on one person but kill another: so the results are R&G to speak). This adds difficulity on creating a 100% antidote, as cancer is linked to a person's own cells (the genetic code varies based on person, it remains a complete mystery if the treatment will work).
The goal of cancer treatments is killing the cancer cells but (ideally) leave many healthy cells as unharmed (if possible), that's easier said than done. Most treatments affecting cancer cells are also going to readily affect a person's healthy cells too (that in itself causes more pain than comfort they would rather die than suffer). Current treatments only provide a working solution, but there's always a chance of resurgence post treatment.
Can Nanobots, mRNA or CRISPR guarantee a 100% cure with absolute zero resurgence on cancer: since how would you know if the patient's body will reject it immediately (even if you knew the person's blood type?). The true instigator of cancer is difficult to answer concretely apart from listing risk factors, since every human being in the world is different (from varying genetic code, health status, pre-existing conditions and etc).
Well, it comes down to what cancer is.
It's malfunctioning cells that are your own.
You go into remission and that means that there is no longer anything detectable.
When it recurs, it isn't the same cells. It's other cells that are now malfunctioning, often in the same way for the same reason.
Recurrence is essentially impossible to prevent unless the original cause is able to be completely eradicated and is something that didn't cause any other changes.
Let's pretend there's a chemical called cancerade. You drink it, you get a tumor in your throat. But, only one tumor, and nothing else changes. You could then kill that tumor and be no more likely to get another cancer than you were before you drank cancerade™
Alas, that isn't how cancers happen. They aren't that simple. Usually, cancerade™ is something that effects the entire system and just happens to cause a tumor or other cancer as a part of its overall damage. So, you kill/remove one, and there's still damage sitting there ready to cause the next one.
Some cancers, you get lucky, and they really only form in one type of cell, and you can generally end up where there won't be recurrence if you remove that type of cell entirely. That's why radical mastectomy essentially does give you 0% recurrence. Which isn't really 100% the case, but it's close enough for government work. Same with testicular cancer, or other similar ones.
The thing is, you can also get cancers in those types of cells from causes that will damage other types. Like radiation. It fucks everything up, so while you might be prone to one type being the first, you don't get a guarantee you'll not have a different kind.
The only way to end up with 0 recurrence is for someone to figure out a way to constantly detect cells as they go wrong and trigger their death individually. Which is what the body already does, it just isn't perfect.
You can't genengineer that. You could get rid of proclivities towards cancers, but it won't fix damaged DNA.
Nanbots? Eh, maybe if they ever actually exist. Right now there's nothing that could reasonably be called that and work in the sci-fi way it would take.
My take is that it isn't hard to permanently cure cancer, it's impossible. Best we can hope for is catching it before it grows.
Not quite true. Cancer events happen weekly and are countered by the immune system.