Timpsu, Shiftry is first of all outclassed by Mega-absol as a Sucker punch attacker because of its Higher speed, and by Toxicroak which has better bulk and a nice typing. It is to frail to come in on attacks (Even resisted hits do alot) and relies on sucker punch to take down faster pokemon. Without an SD boost it fails to OHKO just about everything. and many common pokemon are not even 2HKOed.
B Rank: Reserved for Pokemon who cannot sweep through or wall significant portions of the metagame, but can properly fulfill a given offensive/defensive niche. Support Pokemon in this category have flaws that prevent them from doing their job or are setup bait for dangerous sweepers. Pokemon who are partially outclassed by a Pokemon in A or S Rank, but are otherwise very dangerous, may also fall into this category.
and
C Rank: Reserved for Pokemon who can be effective given the right support, but either have crippling flaws that prevent them from consistently executing their strategy or are typically inferior to Pokemon in the above ranks.
Shiftry fits somewhere inbetween these (C+ or B-) because of its major weaknesses that cannot be helped by poor bulk, no sun perma sun to play in and being simply outclassed by other pokemon in the higher tiers.
Before I start, u forgot to add an Adamant nature, the nature increases the damage output significantly.
It is true that Shiftry is outclassed by M-Absol to an extent. M-Absol sports better Attack and Speed, and does hit considerably harder with Sucker Punch. In addition it has pseudo-immunity to status via Magic Bounce. However, I'd like to point out that Shiftry is actually bulkier than M-Absol, naturally and with the extra investment that's provided, making setting up, in my opinion, easier. Shiftry also has a good second stab, taking into consideration that many physically bulky Pokemon tend to be weak to Grass. Although Absol is fast and hits hard, it is somewhat reliant on its only STAB. While Shiftry is in no way fast, it has just enough speed to be a menace to slow walls, and to hit hard against offensive mons. Even more, Mega-Absol does use up your Mega Stone, so there's that. While I do believe that M-Absol is the better option, I think there should always be alternatives, especially when we consider the roles different Mega Pokemon have in teams.
I think Toxicroak is the better Pokemon to compare Shiftry to. First of all, Toxicroak is not bulkier than Shiftry ( (90/60/60) vs. (83/65/65) ), it might actually be a bit frailer overall. Although completely honest, the difference is miniscule and safe to say they are both frail mons, and that extra bulk Shiftry has is there for a reason. The typing difference is a peculiar one, but I don't believe Toxicroak has arguably a better one. The way I see it, Toxicroak and Shiftry excel against different Pokemon. Shiftry can do much better against the likes of SlowBrothers, Hippowdon and Gastrodon, while Toxicroak is the better answer to Suicune, Blastoise, Umbreon, Heracross and Chesnaught. They both can also take on the M-Aggron + Florges duo quite well. The problems you described (eg. inability to come in, reliability on Sucker Punch, absolutely needs the boost) are in my opinion as much Toxicroak's problem as they are Shiftry's. Ofc, Toxicroak has that wonderful Water immunity, but then again, Shiftry is immune to Psychic. They both take heavy hits from even resisted attacks, and need some support in order to pull the SD. Toxicroak has Gunk Shot, which is somewhat strong even without a boost, but really, they are both quite dead weight without an SD boost. I don't see the reliance on Sucker Punch against faster threats that bad, it is just so strong. Ofc there are guessing games against, for example, Substitute Chandelure, but it isn't a problem that only Shiftry has. The reason I brought up Toxicroak extensively is that I believe that Shiftry and Toxicroak essentially share similar weaknesses, risks and need similar support to function. They are only separated by their typing, which bring their own pros and cons, and one can arque which is ultimately better.
Honchkrow is a good comparison to Shiftry as well, although I need to stress, +2 Sucker Punch is just so incredibly strong that Honch really can't compare to that, not without some Moxie-kills under its belt. Honchkrow does hit harder right off the bat, but to be truly scary, it needs to kill stuff, and with smart play, that can be avoided, whereas Shiftry really only needs one window to pull the SD. Also, Honch is generally slower and just as frail as Shiftry, so it is as reliant on Sucker Punch as Shiftry is. The way I've seen it, is that Honch needs to come to play late-game, because it can't afford to lose the boosts it gathers. Shiftry, on the other hand, can in my experience work as a mid-game wallbreaker as well as a late-game sweeper. It is good to back Shiftry up with a strong Scarfer, for example, that can finish the targets Shiftry has softened, or vice versa.
It is true that many lower-tiered Pokemon can be scary after setting up, but none of them has a unbelievably strong priority like Sucker Punch. Ursaring can use Quick Feet, but it can easily be killed with a Scarfed 'mon. Zangoose has Quick Attack, but I don't think it's fair to compare Quick Attack to Sucker Punch.
To be fair, I don't feel that strongly for that specific
A- rank, it was mostly to start some discussion. Somewhere in
B sound just as good. Although I do have to question, if my comparison with (Toxicroak and Shiftry) is a correct one, I believe they should share a rank, or atleast be at consecutive ranks. If it is deemed that one has a better typing to another, I think separate ranks can be justified, but ultimately, they play very similarly. One thing that troubles me is the part about
B-rank: Reserved for Pokemon who cannot sweep through ... significant portions of the metagame. I have to argue that Shiftry is perfectly capable of sweeping a great majority of the UU metagame. If it is deemed B-rank worth, it should be on the grounds of being outclassed to something, for reasons to be discussed.