To be fair, most single hub DIY residential systems have a limit of 200 devices or so.
That includes Home Assistant Sky Connect, Vera Plus, Homey Pro, HomeKit, Lutron Caseta and others. (IKEA dirigera is limited to 100.)
Is there a maximum number of devices for Sky Connect - #8 by Hedda - Zigbee - Home Assistant Community
Zwave itself prior to series 800 had a hard limit of 232.
But most of the time, the issue is the amount of memory required. And the dollar cost that many consumers are willing to pay.
There are some exceptions. Hubitat doesn’t have a fixed limit, but there many users find they do have to go to multiple hubs once they get over around 200 devices. It does depend on the specific models being used, though.
Homeseer can handle thousands of devices, but only when it’s running on a powerful PC and at a price point much closer to $1000 than 100 for the base system.
Control4 Can also use its proprietary system to handle thousands, in a distributed network with multiple controllers. Again, though, that’s a much more expensive system.
And of course, Lutron offers different systems with different structures, some intended for commercial use, which can also handle thousands of devices, but again at a much higher cost.
(And again, I would note that the old Zigbee ZHA profile was much lighter in memory requirements than the current Zigbee 3.0, so if you find old Internet posts with people talking about Zigbee networks with 400 or 500 devices, look to see if you can identify the profile. It’s probably not Zigbee 3.0.)
So are there alternatives? Maybe, it depends on the exact devices that you have. But most of them will end up costing a lot more, and will probably require running either on much more powerful hardware or on multiple hubs.
speaking just for myself, it’s not the 200 or 300 device limit on SmartThings that I find objectionable, but rather the lack of documentation about it and the continual changing of the guardrails without notification.