Having owned several, shot, loaded for, and killed several deer with the .30/30, and being a career Conservation officer having seen a multitude of deer shot with a .30/30 including other game to include a bear or two, I have a theory (explantion?) on why the .30/30 is occasionally (frequently) "snubbed".
BULLET CONSTRUCTION !
What other posters have said is all true. However, because of how many guns and people have used the gun, and mutitude of purposes it has been put to, some bad ammo has gotten out. Not that it was "bad" but it was not suited for its actual use:
Example......
A friend was using his '94 Trapper (16.5" bbl) as a short range "brush" gun hunting deer in west Ga. Most deer shot were does, going 100-140lbs. He was encountering a lot of "lost" deer, with them running off with good hits only to be lost in dense pine "regenerating" cut-overs. He approached me for some "heavier" bullets for more "smack" knowing I was a bullet caster and reloader. I asked for what he was using them for. I immediately told him he was going the "WRONG WAY"! I recommended for him to use a 125gr Sierra HP, over a specified powder and weight (H-322). He loaded up some bullets I gave him, and got a chance to shoot a deer two days later. He called me and related that the damage inflicted was greater than that he got from his .25/06 and the deer was the first he had ever dropped in its tracks with a heart/lung shot.
To get to the point, most all of the 150gr and 170gr bullets are constructed too heavily for most of the deer and antelope for which the .30/30 is primarily used for.
My older brother spent a lot of his Air Force career in Alaska, Maine, and Montana. There he saw a lot of Moose, and Bear that had been shot with the .30/30, primarily with 170gr Rem Cor-lokts and Federal/Nosler 170gr Partitions. He thereafter has used either a 125 Sieerra, 130gr Speer or Hornady 150gr bullets exclusively for deer as these are the "softer" bullets. This past year he shot two deer moments apart with a chrono'd 2,300fps load with a Hornady 150gr SP from his vintage '94. One penetrated over 24" of deer before stopping against the pelvis from a frontal shot, the other also a frontal shot with a slightly different angle, completely penetrated the deer exiting the far-end offside hindquarter after penetrating over 24" of deer. Needless to say the .30/30 is more than adequately powerful for deer, it's just the bullets often fail to adequately expand due to them being made for purposes besides the killing of lightly built deer.
Kinda like saying that a Tractor is no good for SCCA sports car racing.....Duh!, it ain't built for dat!
Use proper bullets and the .30/30 is useful for most anything you can imagine using it for, from gophers to grizzly even Whales! (though definitely not a first or second choice for either)
My brother got a newfound respect for it while investigating a self-defense kill on a grizzly on an A-F base in the early'80s. Fella had killed a Moose and had gone back next day to finish packing out carcass. A 700-800lb Griz had found it and took offense at being disturbed. Two quick shots from .30/30 ended the argument with one shot (second fired) hitting griz in nose and exiting back of skull. First shot fired at charging bear had struck neck left of cheek and penetrated length wise lodging in paunch of Griz! and would have been a fatal shot, but would have taken a few too many moments to take effect. Ammo was Federal premium 170gr partitions. Hunter related he could never have made either shot with the .338 WinMag. he had used the day previous to kill moose as gun didn't "handle quick enough!". He also talked to some of the local's (native indians) who used an old rusty '94 and whatever ammo was around for killing whales and seals during seasons. He also watched a fella shooting prararie dogs one day while in Montana with rusty old '94. We ended up going back later and helping out with killing a few too! (Sorry, my .257 Robt. is much better P-D gun!)
End of long story, use correct bullets/ammo.
I can't imagine how much better the .30/30's reputation would have been if all these years it had been loaded like the newer Winchester 150 extra power load has been. What few game I've seen/heard of shot with this load, reports have been ecstatic. (150gr @ 2,350fps from 20" bbl, 2,450fps from 24"bbl)
We'll probably be having this same discussion go around another 100yrs from now !