Practical problems with Web applications
Dependency on a Internet connection
Offline operation can be surprisingly usable (e.g. writing an email), but in most cases if you lose your network connection Web applications become useless. Network connections do tend to go down and if a Point of Sale (POS) Web system is offline then you can lose a ton of money.
Making Web applications distributed is very hard. Having a server on local client sites is asking for a whole world of pain. Making that local server update to a remote server with Debian is fairly easy, but that solves only part of the problem. Syncing databases and data. Nightmare.
Implications are that environments without good stable (and cheap) Internet connections (Islands, entire countries) can be (initially) poor areas to market a Web application solution.
Dependency on a Web browser (UA)
UAs usually require quite a good machine to run on. I forget what the minimum specs are for Firefox. For sane operation, you do need at least 800mhz and 256megs of RAM. There are quite a few PCs out there that don’t meet that specification. UAs also tend to require a mouse and hence more desk space compared to terminals.
There are loads of security problems associated with UAs nowadays. UAs are as complex as the operating systems beneath it. In fact, one could argue UAs are operating systems themselves.
In these cases I would suggest clients to switch their machines to LiveKiosk. Send clients new CDs for each major update.