I wouldn't call one art harder than the other by openness of rule set. The level of technical mastery need for boxing takes years upon years to master. Same goes for MMA. I wouldn't say a jack of all trades is harder than being a specialist or vice versa.
But the THEORY is that nothing prevent you from being a Boxing/BJJ/Wrestling/MT wizard all at the same time in the same athlete
On this "theory" level MMA is much much harder
Reality is different
Is unlikely (read close to impossible ) reach absolute peak in every MMA area, so is unlikely get close to the "perfect" MMA style
Of course a "perfect" Boxing style is'nt easy feat either, but you get what i mean
True difficulty of boxing compared to MMA is more about the waaay higher competition level due fuckton of factors
That and the fact that boxing more limited rules/arsenal make harder to just counter-style your superior opponent with something completely different
In boxing if you're getting outboxed, you should still find a way to outbox your opponent.
Of course strategy and counter strategy is a thing, but options don't give you much escape from direct skill vs skill on the same field, and that field got a very high level competition
In MMA you can try to switch to grappling as "easy" solution (is'nt easy)
Basically you have less rock-scissor-paper effect in boxing