Groudon still has many notable advantages and the contest for a teamslot with Landorous-T and Ground-Arc isn't as stiff as it seems. Sun is still good considering how easy it is to use Ho-oh, and Groudon can usually pivot in a couple of times and set it up. Groudon can still phaze physical sweepers such as E-killer, SD Ghostceus, SD Ray, something Landorus can't. Its physical bulk is much higher than Landorous-T's, who is dependent on its ability to make up for it, which isn't reliable in all cases. Fire moves are obviously something that sets Groudon apart from Lando, but there is also Thunder Wave that can support a lot of bulky sun offense teams as well as stopping greedy Xerneas. Lando can't really boast about being hazard resilient that much anymore, since spikes are rare- sticky web immunity is nice- but if we assume SR on your side of the field then Groudon is actually more hazard resilient. Ground Arc has incredible problems, firstly coming from that a support set is inherently Ho-oh weak, an SD set doesn't provide much support and that it is an Arceus-forme with opportunity cost just like any other. Which is not necessarily a bad thing, if it fits your team, it fits, but there are many useful Arceus-Formes this gen who runs support sets easier and better (Water, Rock, Grass, Psychic). I don't think one should approach the problem of using Groudon with: "why should I use it over Landorous-T or Groundceus?" (I don't think you meant this, but it kinda sounds a little bit like that in the OP, and I personally know you are a big fan of both mons), rather I would look at Groudon's physical sweeper checking ability, higher stats, presumable actual higher hazard resilience, support moves etc, and look at a situation where maybe Lando could be more benefitial for example (higher speed, u-turn, better Groudon check itself).