Mono-Psychic teams have a couple major problems - most of them are Pursuit weak, and other than Girafarig, Jirachi, Metagross and the unreleased Meloetta, all of them are Ghost-weak. Some of the biggest threats in the meta use one of these types. CBTar can eliminate a huge portion of a team by just sitting there and pursuiting until everything's gone. SubDisable/SubSplit Gengar outspeeds every Psychic except Zam and the Latis, and even then, the Latis can tie him at best unless they're scarfed.
Both Deoxys-S, Starmie, and (uncommon in OU) Azelf outspeed Gengar. Gengar has to risk things living a Shadow Ball and then OHKO'ing with a super-effective attack. Being Ghost weak isn't a liability because it is not a very common attacking type in OU (bar Reuniclus and Gengar). CB Tar is a hurtle, but every type has a poke that has a hurtle. I'm sure CB Tar won't enjoy Focus Blast's, Super effective attacks from the Steels/Psychics, our good fighting mons (Gallade and Medicham) and the other moves psychics have (Surf Stamie, Brick Break Victini, Superpower Deoxys)
Excellent answer. I like the idea of having multiple options of playstyle. How do the types you mentioned accomplish this?
1. Psychic I already have mentioned in my post above, but the main points for Psychic are the large amount of OU viable Pokes it has, the variety of support moves it can run (thus playstyles), and the large amount of secondary typings it has. Overall I think this is the best mono-type we could do.
2. Steel is very similar to Psychic. While they don't have the very best offensive typing, they're secondary typing options and large amounts of support moves, along with the large amount of OU-viable Pokes makes it good. Steel can run play styles other mono-teams can't, stall, since weakness are patched up easily. (Psychic has Trick Room)
3. Grass can fair a lot better now thanks to the introduction of Ferrothorn. Ferrothorn can act as
the defensive core "alone" pretty much. Grass has a lot of niche moves that other type don't, most notably Spore and Leech Seed. It has a few good physical attackers and great special attackers. It has some pretty bulky pokes such as tangrowth, virizion, roeselia, and cradily but Grass is a terrible defensive type alone. The only problem is that it can get ripped apart
really easily
coughInfernape
cough and some lack of diversity.
4. Ground has great STAB along with a huge amount great physical attackers and defenders. Ground has a lot of secondary typing options along with support moves. Unfortunately, its special side is lacking but the very few options are good ones. Notably Nidoking as a special attacker and while the special defenders are Gastrodon and Claydol. It can cover its weakness pretty well, but not as well as other types.
5. Poison has access to a lot of support moves; most notably toxic spikes, spore, clear smog, and Toxic(sort of poison?). Poison is able to cover up its weakness really well since its a defensive typing and its weaknesses can be patched up easily. The big downside to Poison is the type itself is not that great offensively and we will be guaranteed to use a handful of Pokes since
a lot of Poisons are not OU-viable. Like Ground, the few options are good, but they will pretty much be our only options and the playstyle will be balanced with some toxic stalling.