I presume everyone here is aware that the ultimate end-game goal is to have 6 mono-element team and grids.
How you get there, doesn't quite matter isn't it? The journey for monogrids and rainbowgrids user take to get there is more or less the same. Monogrids player will slowly build their team one grid to another grid. Rainbowgrids will slowly level up their SSRs weapon one element to another element.
If we're talking how fast you get there, it really depends on your luck with gem gacha/weapon cave. The more time you can sink in the game, the faster you'll get there.
If we're talking how easy each content is, I'll say monogrids will struggle when they do not get elemental advantage/get elemental disadvantage, which rainbowgrids will probably not experience. But if both monogrids and rainbowgrids have equal playing ground (same hime quality, same weapon quality, same eidolon quality), monogrids should have a better damage output when compared to the rainbowgrids, because of the stacking of the same element SL that rainbowgrids will not get.
Also FYI, I wouldn't call someone that uses 10 mono-element weapon + 1 off-element weapon a rainbow grid user. I'd classify a rainbow grid user as someone who has less than 50% of his weapon grid in 1 element, anything more is just mono element user building towards a monogrid.
So, to say which grid is better, that's a hard conclusion to draw. Both grids have their pros and cons. Anyone will have to think carefully of their own circumstances (playing time, etc) before adopting an approach.
I'd say rainbowgrids rely less on RNG and event timing because your team will be decided by who you got from gacha and event timing wouldnt matter for you because you just farm them anyway.
Monogrid will need the right element himes and right event timing to start building their team. It's not going to be a good time to be a fire user (for example) if the next 4 months events are going to be a non-fire event and so on. If you cant access event SSRs, it can put quite a bit of delay on your progression towards getting a monogrid.
With that said, aside from of some ragnarok difficulty content and perhaps the tier 4 accessory (do correct me if i'm wrong), monogrids can clear those content with disaster drops anyway, so its not like you're going to be seriously hindered from getting to the end goal.