Devices being marked “offline” is a long-standing problem with SmartThings and can happen whether you have 3 devices or 300. Many forum threads about it. Is that what you meant by “losing connection”? If so, it’s an artifact of the cloud platform, and nothing to do with any particular protocol. The hub itself hasn’t lost connection to the device, but your cloud account thinks it has. 
If you meant you actually did lose connection, that would be a different problem, most often caused by one bad repeater somewhere.
As far as limits…Any zwave network, regardless of hub brand, is limited to 232 devices total. Zigbee allows for thousands. There’s a FAQ on this:
FAQ: Hub limitations on Number of Devices?
There’s another limit that affects the number of zigbee child devices that can attach to one parent, but that wouldn’t affect plug-ins.