I feel like ME3 was an excellent game up until the final moments... it's amazing how such an ending soured the overall experience for me.
I found especially the first two missions egrigious but mostly just Earth. Now, it gets the basic points across, that is the themes: We cannot fight alone, we must band together, war is hell, people must be saved. That's all good but the continuity is not good. I like that they reflect on whether you played Mass Effect at all before, whether your played only ME1/2 or if you played Arrival but a lot of the flavor dialogue there is really bad continuity of characterization.
The tone of ME3 moreso than ME2, even when you run with an import-save is that of "As you know, EXPOSITION ABOUT STUFF YOU ALREADY KNEW", and not even dialogue you'll hear if you don't have a save import. It's like BioWare fixated so much on the new player first design that it seeped into even their "old time player" writing. It's shit like Anderson in seriousness asking "Is this (the Reapers) what Shepard warned us about?" In ME1 he was the guy who put the whole Reaper myth on us and took it so seriously, and he's now asking as if he's not even sure what it is? Then there's the "defense comittee" that is bad no matter if you played previous games or not. Like, they all look like they're about to cry and all they ask is "what should we do!?" to Shepard, there's just zero realism. The higher ups of Earth's defense is facing an unknown threat and they don't even have any of the expected gruff military "We must face the enemy!" or anything? The whole scene is played up for silly melodrama in a really patronizing way, and then when everything is already kind of egrigous you get the scene with the Vent Kid, then you get a scene where Shepard says completely stupid things like "We can't leave Earth, the fight is here!" as if he hasn't already seen how unstoppable a single Reaper was. (and there's implied to be thousands of them just within a hundred miles"
And on Mars you get stupid conversations with Ashley or Kaidan acting like schoolchildren about your time spent with Cerberus even if you resolved that Horizon encounter pretty mutually in ME2, and they also portray the new centerpiece of the plot, the Crucible in this matter-of-fact "Oh by the way, we had this weapon that will defeat the Reapers all along!" way, really nonchalantly.
But yes,
after that the real ME3 story takes over and becomes mostly really good. Everything from the Turian Moon mission, the side-plots, the Citadel and all the way to, I would say, Cerberus HQ the plot is quite strong again, even though there's occasionally a sense of "What are we actually doing this for?" (because the Crucible is never developed, it keeps being "We don't know anything about it, except MAYBE it can defeat ALL THE REAPERS".
So yeah, I think ME3 is a mess. The central core of its plot is really shitty all the way, and the ending just takes an extra step attempting to introduce some overly misguided denoument, reaching for the series to have some deep and profound message about some IRL issue like "What if our electronics are dangerous to us!??" but it's as if everything
around the central plot is very solid, and it makes for an awkward campaign IMO.