A smart lock is a small motor, a radio and a battery attached to a mechanism that has to work every time. The reliability requirement is what makes it difficult.
Failure has to leave the door usable
Ordinary connected devices can fail by stopping. A lock that stops either traps people inside or leaves the door open, and neither is acceptable.
Designs therefore retain a mechanical path, usually a key or an interior thumb turn that operates independently of the electronics.
That mechanical path is also the weakest point from a security perspective, so the physical cylinder still determines much of the lock's real resistance.
Battery state becomes a security property
A depleted battery on a speaker is an inconvenience. On a lock it can mean being unable to enter, so warning behaviour has to be conservative.
Motors draw substantial current briefly, and cold weather reduces available capacity, so a battery reading as adequate at rest may fail under load.
Emergency power contacts on the exterior exist for this reason, allowing the lock to be energised temporarily from outside.
Radio through a door is not a solved problem
Locks sit inside metal doors and frames, which absorb and reflect radio signals badly. The mounting location is chosen by the door, not by where the network happens to have good coverage.
Low-power mesh protocols are common because they tolerate weak links better than Wi-Fi and allow mains-powered devices nearby to relay messages onward. That introduces a dependence on those relays continuing to exist and stay powered.
Battery constraints compound the problem, since a lock cannot keep a radio listening continuously without draining itself. It wakes on a schedule, which is why commands sometimes take several seconds to take effect.
Credential management is the real feature
The reason to connect a lock is not remote opening but the ability to issue, schedule and revoke access without cutting keys.
That requires the lock to hold a current list of valid credentials, which means revocation only takes effect once the device has been reached.
An offline lock with a stale list will still admit a credential that was revoked centrally, so synchronisation frequency is a genuine security parameter.
Retrofit and replacement solve different problems
Retrofit units drive the existing interior thumb turn and leave the exterior hardware untouched, which preserves the original key, the original cylinder and the appearance of the door.
Full replacements integrate the motor directly with the bolt and generally handle alignment better, since a retrofit must overcome whatever friction and misalignment the existing door already has.
That friction is the most common cause of a lock that works on a bench and jams once installed. A door that needs a firm pull to latch is asking a small motor to do more work than it was specified for.