All detectors have a learning curve, but some are longer than others. Many detectors are easier to use for someone who never had a detector than they are for someone switching from another detector.
The reason would be that in addition to learning the new detector, one must forget some things about how the other one works. The same object may give different ID # or different tone, different sensitivity numbers for best stability.
Some people learn some things fast, some learn everything fast, or learn slow. Field testers are usually people who can adapt to any detector almost immediately, so they get the maximum results in a short time.
When someone says a detector has a long learning curve, what detector are they comparing it to? Did they read the manual? Do air tests? Plant coins?
Also, long learning curve can mean it takes time to know *everything* about a detector, or to get best results. This of course does not mean they won't find anything if they don't know everything. Is it is too difficult or not? Reading the manuals on mfrs websites might help decide that. Best wishes, George (MN)