I'll admit to not having read the reviews, but I'd guess the main focus for thing beings repetitive is the tier system, remaking the same spell 4-5 times and then doing it for however many spells you use is going to be repetitive by anyone's standards, largely because of the perceived lack of progress over any actual repetition in what you're doing. I have to say that that is probably my interpretation because recently I have been considering suggesting removing the tier system entirely, back when we had infinite tiers there was an argument for it but as we just repeat the same 5 tiers now it seems sort of like a hangover.
TLDR: Where is there a point in the game where you want to do something fun, but instead the game is making you do an activity you find un-fun before you can get back to the fun bits?
This is going to seem a really strange answer, and indeed not specifically the kind of answer you are after, but the least as it stands the least fun I have in the game is when it comes time to kill the Overlord. I enjoy everything up to and including killing the lieutenants (which I generally do when I have a couple of Tier 4 spells), but when it comes to the Overlord it is all sort of overridden by the knowledge that killing the Overlord makes what you just spent your time doing completely pointless.
As to the real question of where I find there to be grind or drag, it's mostly down to spell choice, I arbitrarily pick new spells to use when I start a new continent, if missions don't offer what I want then that is where the drag comes into it, you can spend a lot of time getting nowhere whilst trying to get what you want rather than using what the game offers you.
I suppose the real problem there is once you are part way down a path switching to a different one is somewhere between difficult and tedious. I mean if all the initial missions give me say Sea Essence and I start upgrading my Water Spells, then suddenly the RNG screws me and no Sea Essence spawns, my options are either...
1) Persevere with Water Spells and grind secret missions or advance time a lot in hope that some turns up.
2) See what resources are spawning and start again from scratch with those spells whilst hoping that the RNG doesn't Screw me twice.
Neither of these are particularly fun options. and the second whilst the most rational choice goes against the grain, it's like I am giving in to the system not going along with it.