Comparing catalogs, Plejd focuses more on lighting control, while Shelly has a wider range of meters and switches installed in electrical cabinets. The similarities lie in those basic relays that switch loads on/off based on defined scenarios.
The more versatile control/management of the Plejd system seems to require the purchase of a mesh gateway, whereas Shelly’s relays and such can be connected directly to a WLAN network. (Excluding Bluetooth dongles or other limited special cases like ZigBee, etc.)
With Shelly, expansion scripts can be entered into the app; in Plejd’s case, they would likely need to be entered through home automation software, which the gateway then passes to the devices. The solution is likely carefully considered.
In summary - Plejd has, in my view, tried to “Apple-ishly” trim away the engineering clutter from its products, leaving those more interested in tinkering as Shelly customers. I would guess this also appeals to electricians - the simpler the installation and the more limited the (customer’s) potential for fumbling, the better.
I’ve asked a couple of installers for comments on Shelly installations, and the response is something like “if I absolutely have to.” There has been a slight fear of acting as the customer’s IT support if some control doesn’t work. In electrician training, there is (presumably) a separate IT-electrician track, and without a personal hobbyist interest, new “smart relays” cause uncertainty about one’s own skills and the durability of the devices. For some, it is important to have the products on the lists of wholesalers like Onninen or similar; I assume this facilitates billing and warranty replacements.
Disclaimer: I have a couple of Shellys installed in my household, controlling a few LED strips on/off. Since some simple Shelly relays operate on 12V voltage, an electrician is not needed for the installation. At the same time, I would point out that these WLAN-connected IoT devices should be isolated in their own network, with appropriate restrictions set for traffic between networks.