Material choice, braided stainless outer versus a plain rubber hose, changes the durability picture too, and is worth its own consideration alongside bore, length and rating.
Pressure rating is where corners genuinely should not be cut. Every flexible hose has a working pressure it is designed and tested to, and mains-fed systems, unvented cylinders in particular, run at higher sustained pressure than an old gravity-fed tank system ever did. A hose rated for low-pressure use has no business being fitted on a mains-pressure job, regardless of whether the thread happens to match.
Length matters just as much, and not only for reach. A flexi hose that is too long for the gap it is bridging tends to kink or sit under unnecessary strain, both of which shorten its working life. Too short, and it is either fitted under tension or simply does not reach, forcing a compromise on the install that causes problems later. The right length sits with a gentle curve, not a tight bend, and toilet replacement parts supplier not stretched taut.
A thermal cut-out is a safety component, designed to break the circuit if the unit overheats for any reason, whether that is scale build-up, restricted flow, or a fault elsewhere in the unit. A cut-out that has tripped and stayed tripped, or one that trips repeatedly, is a common reason a shower stops working even though the heating element itself is fine. Trade buyers restocking cut-outs are usually replacing a component that has done exactly the job it was designed to do.
For a trade buyer troubleshooting inconsistent hot water on an otherwise healthy system, checking the heater's rating against the cylinder's actual capacity is a faster diagnostic step than assuming a fault with the element itself. Matching the two properly, rather than replacing like-for-like from whatever failed, is usually what actually resolves the complaint.
Approval marks are the shorthand for all of the above. A hose carrying recognised approval marks has been tested against defined pressure, temperature and material standards, which takes the guesswork out of matching a part to a job. For a trade buyer working through multiple jobs a week, checking for the mark is faster than re-deriving the spec from first principles every time, and it is the difference between a part that is fit for purpose and one that merely fits.