Excadrill should be a B rank mon. It's not very good by itself honestly. It has a hard time pulling off spins unless it sacrifices itself and it's way too slow in general. The offensive sets are outdone by both Garchomp and Landorus-T which are both top 5, making it kinda miserable, especially since they both beat it 1v1. Not even outspeeding Adamant Garchomp is terrible
I disagree with Exca being as low as this (imo, it should be a sub-rank higher than B); sandstorm is really good even without it, and the ability to just flat out shred it (good on sun balance, for example) is hugely important for a large number of teams, while it also pairs well with various pursuit-weak 'mons due to its ability to flat out take advantage of the best pursuit trapper in the tier (t-tar), discouraging it from coming in. Also, something that is relatively unexplored and effetive atm is bulky driller. I've been testing out a variant of the ubers bulky set and I've had good success with it.
Excadrill @ Leftovers / Air Balloon
Ability: Mold Breaker
EVs/Nature: See description for some viable spreads
- Rapid Spin
- Earthquake
- Swords Dance / Iron Head
- Substitute / Stealth Rock
Mold Breaker allows it to not get walled by levitators (namely Rotom-W switch-ins). Sub is great as it allows it to take advantage of switches it forces v.s. stuff like Tran, Magnezone and Rotom-W and it is capable of blanket checking a large number of special attackers due to its decent natural special bulk (for example, it beats Forretress in the special bulk department); it fits well onto bulky offense and balance, and it is a very capable 'mon when facing powerful special hits. I chose adamant over careful despite adamant allowing a higher attack stat as it improves Excadrill's overall bulk. There are lots of different things you can do with the EVs though, making it customisable to the team's needs: running a spread of 172 hp/12 atk/252 SpD/72 spe with a jolly nature outpaces jolly scizor and lets Excadrill make 101 HP substitutes; a spread of 216 hp/12 atk/252 SpD/28 spe with a careful nature outruns uninvested lando-t, hits a leftovers number and increases its ability to take hits at the cost of power and speed; 120 hp / 8 atk / 252 SpD / 128 spe with an adamant nature takes the first spread and exchanges a little bulk and speed and the ability to make 101 HP subs (EV'd to live specs surf from latios after rocks and outpace jolly t-tar) in exchange for a little power.
The thing about this set is that it can take advantage of a reasonable pool of pokemon due to its ability to take special hits decently well, combat Rotom-W and act as a shaky check to Lati@s and Hydreigon, and it can blanket check any special attackers lacking an SE hit reasonably well.
Also, I wouldn't discount its more offensively-inclined sets. While it may not have the momentum-grabbing ability and power of Landorus-T, and it may not have the god-tier typing or contact-punishment of Garchomp, but its ability to punish sandstorm, spin v.s. non-Focus Miss Gengar (unless under sandstorm or holding choice scarf, in which case it beats Focus Miss variants 1v1 and has free spinning power for the rest of the game) and (if Mold Breaker) beat Rotom-W 1v1 gives it good utility and niches over Lando and Chomper. Also, its neutrality to ice allows it to check BoltBeam users - most notably HP Ice Magnezone - better due to the increased number of switch-in opportunities v.s. such pokemon.
Also, I think you are underselling Exca's ability to spin. Sure, it isn't the best spinner around by a long-shot. However, it does force a large number of switches and, due to the meta not demanding Rock Slide like ORAS' meta does, it has space to run both SD and spin; this allows it to be more threatening in the spinner role (the ability to not only take advantage of Tyranitar, but to also set up for a sweep and to remove hazards for things like QD Volc is hugely valuable) and it allows it to fit onto a far greater number of teams to support 'mons such as pursuit-weaks, fire-types (and by extention sun 'mons) and other rock-weaks like Volcarona, Kyurem/KyuB, Tornadus and DNite.
I respect that it has flaws (pretty cookie-cutter when it comes down to its coverage options, not as consistent as chomper/lando, struggles to break through certain 'mons, has to choose between losing to rotom-w, gengar or fastmons), but it is reasonably versatile when it comes down to spreads it can run and the roles it can fill on a team.
I think that B+ is a better representation of Excadrill's qualitites in the metagame than B-, as it is definitely the best weather-punisher in the meta atm due to its great typing and good utility on a variety of teams. However, Mold Breaker sets are also perfectly viable for their ability to beat things which it and its ground brethren otherwise loses to, such as Rotom-W, and it does have a fair share of flaws which I feel mean it shouldn't be in the As.