Putting my support in for Harvest
While it is indeed generically versatile, Harvest has several problems that specifically make it balanced without being overpowering.
Harvest is only *consistent* in Sun. While it is indeed very possible to switch into a Scald while it is raining and have Lum Berry take care of the burn, there is no guarantee Lum Berry will grow back, meaning if you were relying on LumRest as your means of tanking foes, you are banking on a 50% shot to regain it outside sun. If the Scald user is faster and they burn you, they will eat up your Lum Berry again before you Rest, leaving you vulnerable. Furthermore if you do actually LumRest, it is again another 50% shot to regain the item after using Rest. Should you be forced out, ordinary sleep rules apply and you are banking on another 50% shot to bail you out.
Harvest has a few other problems that keep it from being overpowering. This stems from 3-4 moves and an Ability. The moves are Bug Bite, Pluck, Trick/Switcheroo, and Knock Off. Due to their impracticality I don't think Embargo or Magic Room will be used, and Incinerate is far too weak to take seriously.
Bug Bite is the biggest threat, and it happens to have great distribution
for being a BW2 Tutor. The vast majority of these threats are in the "Bug" Egg Group, meaning we would be increasing a fairly unused type, or at least promoting SD rather than CB Scizor, as it commonly uses Technician Bug Bite as its STAB.
Pluck is not as effective as Bug Bite because the weakness is only 2x, but it also has decent distribution
on the basis of being a TM and the flying types that get it tend to have beaks or buck teeth (or are Crobat and Sigilyph).
Trick of course can be possessed by the Latis themselves, but if Specs is tricked away from Lati@s then we've made them easier to handle, even if they've made life more difficult for CAP 5.
Finally there is Unnerve, which prevents berries from being used altogether. While I sincerely doubt Tyranitar or Haxorus will start switching their ability, there exist several Pokemon on the list that we would like to encourage, specifically Galvantula, Aerodactyl, Houndoom, and Vespiquen.
3. Fulfilling the concept.
Harvest's primary versatility is defensive
versatility. With very limited resistance to priority, the best applications of Harvest I can see are with 4 primary berries: Lum, Resistberries, Sitrus, and Enigma Berries, with possible runners up going to Jacoba and Rowap, and perhaps a tiny niche for Chesto Berry as it ensures a random fast Thunder Wave or Toxic don't mess up your Rest plan. Harvest is never going to help CAP5 sweep effectively unless we give it either sweeper-level Speed and Attack/SpA or very strong stat boosters. Of the scenarios I can see, we can most certainly tie this Pokemon at the hip to sun to maximize the offensive potential of this defensive ability.
The Resist Berries also help us deal with key Pokemon like Keldeo (with Chople Bery) by giving us a chance to survive their onslaught and put them in KO range for another Sun Pokemon. It helps us win more neutral matchups with things we want to threaten while doing very little against types we don't. Occa Berry is not going to save us from sun-boosted Heatran Fire Blasts, as we will either play in Rain and rely on an inconsistent Harvest to keep procuring a method of survival, or the more likely action on such a team which is use a Water-type. Yache Berry is thus the most likely to be used berry, or perhaps even Enigma as it allows us to better tank Ice Beams that do 49% damage or less. Either way, the ability allows us to either Switch In
or Stay In
- which is exactly what we want a pivot to do.
I think this makes Harvest the best ability to shape the rest of our stats and movepool process around, because it allows us to fulfill the concept by making the consistency benefits of play in sun far outweigh the generic utility of Harvest when it works 50% of the time in conditions that can be played around or addressed by competent competitive strategies.