So, I've been thinking a lot about hard charm vs weakness policy vs focus sash. It's actually quite a fiddly choice. My thoughts... these are somewhat lengthy. I may have missed something here... please feel free to comment/correct.
Focus sash
The advantage of focus sash is that it gives you the best all-round protection against a 1HKO of the three options. As long as Primal Groudon is fresh, every one-shot will be blocked, including G-Booster. This can be simply huge with Primal Groudon because a single attack is frequently 2 prizes. It protects against VirGen (not only G-Booster but also banded Megalo cannon) but also other potential 1HKOs like a maxed Flareon or Night March attack with a bangle. It will do the trick against Mega-Rayquaza too .
Keeping one of your one-prize attackers going for an extra turn can be huge too. It can let you really mess something up with Hawlucha or get an extra Shout of Power off.
The main disadvantage of focus sash is the obvious limitation- doesn't work if the Pokemon is damaged. So bats, sniping and so on will alll get round it. It would be a terrible shame to have this on P-Groudon but have your Groudon sniped for 20 by a Megalo Cannon. If you are serious about using focus sash you probably need Mr Mime as well.
Weakness policy
Weakness policy stops several 1HKOs, like anything with a Megalon Cannon and Leafeon's Energy Crush with 6 energy on your board. Its primary advantage over focus sash, though, is that it performs better against low/mid-range grass attacks. Eg with focus sash down, a banded emerald slash is stil la 2HKO- with weakness policy it is a 4HKO. Energy press with 5 energy on the board is a 2HKO with focus sash, but a 3HKO with weakness policy. Banded "Blockade" is a 4HKO with focus sash, but an 8HKO with weakness policy. The other big advantage is that, unlike focus sash, it still works when Primal Groudon is not fresh. If P-Groudon is sniped with megalo cannon while using focus sash, then a banded megalo cannon (or megalo cannon with Deoxys) is a 1HKO. With weakness policy, Groudon will survive the megalocannon.
A secondary advantage of weakness policy is that it is quite useful to put on Landorus in the toad matchup (if you can). Weakness policy prevents the banded quaking punch doing 100, which is important for surviving an extra turn. This mathchup comes down to whether you can get 4 energy onto a Primal Groudon (if you do, you have won, that is it). So just a single extra shout of power can make the difference.
The obvious disadvantages- being bypassed by G-Booster and not protecting against the big non-grass 1HKOs. With M-Rayquaza likely to be everywhere this soon, this seems a big deal.
Weakness policy can be huge, but if you have to chose between focus sash and weakness policy, I think you would usually go for focus sash
Hard charm
Hard charm is a bit of a sleeper in this comparison- I think it's actually a bit underrated. Hard charm will prevent a 1HKO in quite a few situations. Megalo cannon needs muscle band *and* deoxys to get there. Night March can't 1HKO a hard charmed P-Groudon- its damage cap is 240 (11 NMers in the discard and silver bangle). It's very hard for Flareon to hit 260 damage, and against Leafeon you can still have 6 energy down and not get 1HKOd. It will prevent M-Rayquaza KOing your Primal Groudon. A few things will still get round it- G-Booster and a fully pimped out Megalo Cannon.
Hard charm helps with bench damage, which is very nice. Not from bats of course, but regular sniping like Hammerhead, Overrun and Megalo cannon. Reducing bench damage by 20 is a big deal. Two hammerheads turn your delicious, super-hard-to-kill 240 monster into a regular 180 HP big guy. Hard charm makes that 220.
The other main advantage is that hard charm helps in most matchups and most situations. While focus sash and weakness policy are mainly about big hitters and decks that hit for weakness, hard charm is for every matchup. The value of it is easy to miss, but the math is such that you often turn a 3HKO into a 4HKO or a 2HKO into a 3HKO. E.g. if Hawlucha is hitting for 120, that's a 2HKO without hard charm and a 3HKO with. If Donphan is hitting for 80, that's a 4HKO without hard charm and a 5HKO with. It's great for getting the damage to cross those lines- 120, 80, 60. And of course it doesn't have the annoying drawbacks that focus sash and weakness policy have (only protecting against grass, or only protecting a fresh Groudon).
Right now I am very tempted to just play spiritomb and hard charm as my only item. Just because it does almost the same work as focus sash or weakness policy, but is just more useful in more situations. If I happen to play VirGen and they have deoxys, well, congratulations to them. I would still actually like to run weakness policy still (because of the Toad matchup) buy in my build there is not very much space for items, and I would like to run enough hard charms to get it consistently.