Bike lock security varies enormously by lock type — a lock that looks substantial isn't necessarily one that resists real cutting or leverage attacks.
U-locks vs. cable locks vs. chain locks
U-locks (rigid, shackle-style locks) offer the best resistance to cutting and leverage attacks among common lock types, since their solid, thick metal shackle is hard to cut with hand tools and difficult to leverage open. Cable locks are lightweight, flexible, and easy to carry, but are generally cut through quickly with basic tools — genuinely useful as a secondary lock for a wheel or accessory, but weak as a bike's sole primary security. Chain locks offer a flexible middle ground, often with decent cut-resistance depending on the chain's thickness and hardened steel quality, at the cost of being heavier and bulkier to carry than a U-lock.
A thin, "convenient" lock is a real security tradeoff, not just a style choice
Thinner, lighter cable locks and smaller U-locks are more convenient to carry, but that same reduced material is exactly what makes them faster and easier to defeat with basic tools. This isn't a minor aesthetic tradeoff — actual bike theft statistics and lock testing consistently show a meaningful gap in real-world resistance between thin, lightweight locks and substantial, thick-shackle U-locks or hardened chains, even though the thinner options look similarly "locked" at a glance.
The two-lock strategy addresses a real theft pattern
Using two different lock types together — commonly a U-lock through the frame and rear wheel, plus a cable lock securing the front wheel — addresses a specific known theft pattern where a thief with tools suited to defeating one lock type may not be carrying the different tools needed for the second. This isn't just redundancy for its own sake; it specifically raises the effort and tool variety a thief needs, which meaningfully deters casual, opportunistic theft even if not against a determined, well-equipped thief.
How and where you lock it matters as much as the lock itself
Locking only through a wheel and not the frame means a thief can simply remove that wheel and take the rest of the bike, since the wheel's quick-release or bolt often costs far less than a stolen bike. Locking to a genuinely secure, immovable object — not a sign post that can be lifted over, or a flimsy structure that can be broken — matters as much as the lock's own strength, since even the best lock doesn't help if it's anchored to something that can simply be removed or broken instead.
The one thing people forget
Check for independent lock security ratings, where available, rather than relying on the manufacturer's own marketing claims about strength — some regions have independent testing organizations that rate locks against real attack methods, and these ratings are a more objective comparison point than a lock's price or branded "heavy duty" language alone.