Not severe, if you know what your circuit is doing. Probably the most useful tool you can have is a dual trace oscilloscope operating in the DIFFERENTIAL mode. Yeah, it takes two probes, which might seem like a pain at first, but no more so than using a DVM. You don't have to worry about ground clips and all that. Because the majority of my bench work involves push-pull R.F. amplifiers, the two probe approach is second nature....well actually FIRST nature now!what are the difficulties in troubleshooting an ungrounded system?