Using computers to record doesn't necessarily cause quality problems in and of itself, but it does introduce a number of variables that you otherwise wouldn't have to worry about:
- Latency: Digitizing audio into a computer involves buffering samples, and then processing them. It is possible to minimize it with good hardware, but as far as I know it cannot be completely eliminated.
- Drivers: Hardware attached to a computer requires driver software, which can be buggy. Not all manufacturers' drivers are of the same quality, and it's often not clear whether an issue is related to your configuration or to bad drivers.
- Resources: Your audio software and hardware have to compete for CPU time and memory, just like any other application. This means that it's possible to interrupt your audio due to other applications using system resources, even when it seems like they shouldn't affect audio at all. This can show up in unexpected ways, such as your audio dropping when your wireless card attempts to find a network (this has happened to me).
- Recording software: DAW and recording software can be very complicated, since there are a lot of options.
- General complexity: There are more changeable parts involved that could come from different manufacturers (interface, usb/firewire, CPU/RAM, hard drive, operating system), so it's a more complicated machine.
There are ways to deal with each of these. If you're monitoring your recording through hardware, or perhaps you don't need to monitor it (such as a podcast where you've already got your levels set and don't need to hear yourself), you can get around the latency issue. Alternately, you could invest in hardware that can handle lower latencies. You can avoid driver problems by sticking to hardware that's known to have good driver support, or that you've been able to try out. You can minimize other applications' resource use by not running other software. And even very complicated systems can be learned.
For what it's worth, I personally use my laptop and a firewire interface for all of my general recording, and it has worked out very well for me - I don't mind the complexity compared to a recorder, since the tradeoff in flexibility is preferable to me. I don't run much non-music software on this computer, and I keep the wireless off during important recordings. Latency is a minor issue, but has been tolerable - I use my interface's hardware monitoring feature when I need to.
As far as specs, the more power the better, but any machine capable of running its operating system with plenty of room to spare should be adequate. Check the recommendations on your interface hardware as well as recommendation software, and add in some extra breathing room. It is possible to happily record on older hardware so long as you're using software from a similar era.