Aye, for croppers you need either a massive chain, or an overhardened one. Engligh Chain Co make a 13mm link chain that’ll beat all but the most massive boltcutters, but at the disadvantage of being very brittle and so rather easy to shatter. Almax and the bigger Pragmasis will stop anything apart from hydralics.
Anything will cut with grinders but luckily they’re relatively uncommon- the reason being, most bikes are chained up with toy locks that thieves can break silently and quickly with boltcutters (or not chained up at all), so why use the noisy, slow approach? It’s just like swimming with sharks, you don’t need to be faster than the shark, just faster than the other swimmers 😉
(most chains and probably all cable locks are as easy to break with appropriate cutters than they are to unlock with the key)
And yep, as zippy says… You can’t secure every component, and it’s easier to cut most frames than a good lock.