It's sometimes outclassed by other Pokémon
If you are looking for info about it in-game, I'd say it's not that good, you can choose Hippowdon as a physically defensive Pokémon as it got access to reliable recovery in Slack Off, but seeing that in-game is almost always about hyper offence, you can go for some other Offensive Ground Type Pokémon, like Garchomp.
If you are looking at it through a competitive perspective, then the situation changes.
It's currently in the RU tier, and also it's A Rank in the Viability Rankings, which is not bad, infact, it's good.
It can function as a Specially-Defensive Pokémon who can also tank Physical hits, courtesy to its godly base 200 Defense, and can also set Stealth Rock, which is another point that makes it good.
It's analysis state:
Steelix's natural bulk, defensive typing, and multiple utility options make it an excellent Stealth Rock setter for balanced or bulky offensive teams. Its solid typing blocks Volt Switch from common Electric-types while also packing a Toxic immunity, something no other Stealth Rock user can offer. Additionally, it serves as a check to common threats like Sylveon, Xurkitree, Toxtricity, Registeel, and Metagross.
However, some Electric- and Fairy-types, such as Raikou, Gardevoir, and Togekiss, commonly carry coverage moves that Steelix is vulnerable to. Steelix also struggles with common Water-types like Seismitoad, Milotic, and Volcanion, and its Grass neutrality makes it a Steel-type that cannot reliably check threats like Zarude and Roserade.
From the above, one can infer that Steelix sports a very good typing, which grants it immunity to both Toxic and Thunder Wave, which is extremely helpful through the course of the match, and it also has a very great natural bulk, which enables it to survive a lot of hits.
But, on the other hand, it has a weakness to a very common type in Water, and sometimes a few Pokémon carry coverage moves that Steelix is weak to, which is a big oof.
But actually, in RU, it's a Pokémon which is not overlooked while team building, and every player should prepare against it, as it can really pose a problem sometimes!!
All in all, in-game its outclassed, while in competitive, it's actually a nice choice in its tier.
Source(es): Steelix's analysis, and I also used it sometimes.
Hope it helped!
