Definitely too busy with clunky and incompatible ideas that don't go anywhere beyond a veneer of "we're going to invade, isn't that exciting?" by the Daleks. Too busy and inconsequential, and for no reason than crowd-pleasing bits. Like "The Chase" but without the canned comedy. Oooh, bring back Davros again, using the same plot points as in his previous story? Oooh, callback to the Movellans and their battles! Why? Who cares, the Movellan canisters are on Earth, we don't care or explore that since why would the Movellans come here or why would the Daleks, when they can bog off to any old planet and tinker with the canisters there and with no pesky interlopers. Oooh, Daleks want to take over Gallifrey all of a sudden! Why, apart from the most epic clip show sequence from the last three years*? Oooh, there's a Dalek time corridor - it's probably related, but the galaxy's a big place so aren't we lucky the TARDIS stumbled into it? "It was but one trap!" exclaims a Dalek, but we don't get explanations of the others, of which some might tie in the story's busy-ness better?
At least the story easily had the highest death count in Doctor Who. Take that, T-800! But in seriousness, the direction and grim atmosphere/tone of the story does elevate it,
if you can get into it. The stories surrounding this one are so much better, though, with less busy storylines that feel like they have more meaning and something to say.
The US transmission had incomplete effects as well, e.g. no Dalek gun zap visuals.
I'm pretty sure the casings weren't able to get properly refurbished due to budget constraints, though I recall they looked better than they had in "Destiny", where they were completely knackered. I also recall that this story was originally to have been at the end of season 20, before 'The Five Doctors'. Called 'Warhead', industrial action cobbled it entirely (and almost took down Terminus too) - add in other hassles JNT had to deal with regarding previous years and that's another reason he wanted Colin Baker's first story to be made, so that Davison wouldn't lose out on his swansong... I sometimes do wonder if this story would have fared worse if industrial action hadn't clobbered the end of the season.
In its defense, Maurice Colbourne and Rodney Bewes both steal the show.
* Except for Leela but, in fairness, with a 21 year-old show, and with no database of every companion the Doctor had, plus the luck of enough existing episodes to get snippets of everyone else from, the one character omission is impressive that there weren't any other accidental omissions.