You can still do unit testing of your client-side code if you wrap everything in components that separate state from rendering. The rendering itself you can only test with a human looking at the page anyway (or by comparing to reerence renderings). Extjs uses this approach. I can easily unit-test my forms written on top of extjs.
Fundamentally this is a bikeshedding debate. Whether you're programming on the client or server, the tools are equally capable. The big difference is the server puts you closer to the data, and the client pus you closer to the user. Either way you're struggling with latency, and you do different trade-offs, and either way you reach your goals.
I used to build stuff on the server, then i switched to extjs, with the server only exposing a bunch of json-rpc web services. It's not as big a deal as people paint it out to be. Extjs abstracts away browser differences well enough that i rarely come across issues that aren't reproduceable for me. If they're reproduceable for me in my browser, they're easy to fix.
Now, extjs is a DSL, it's closer to flex development than to traditional web development. It has a learning curve with a pay-off at the end, and the pay-off is easier ui development. If you're building ui the traditional way, but client-side, ymmv. Still, anything to do with rendering glitches is something you'll have to debug by eyeballing it, regardless of where you render your ui. There's no way of automatically logging misplaced floats.
P.S. There's also less to monitor. Typically you'll monitor for performance and security. Performance depends on the browser and pc specs, not the server's load, and security has nothing to do with the client. So, really, there's just not much to monitor. You can attach error handlers to automatically report exceptions to the server, but i've not had a need.
Fundamentally this is a bikeshedding debate. Whether you're programming on the client or server, the tools are equally capable. The big difference is the server puts you closer to the data, and the client pus you closer to the user. Either way you're struggling with latency, and you do different trade-offs, and either way you reach your goals.