At what frequencies does PCB design get tricky?
Trace length versus frequency - for sending data or carrier waves between one IC and another, the guidelines are fairly tolerant I'd say. The maximum frequency that could be generated in significant amounts (maybe up to several harmonics for a square wave) is the limiting factor and if your trace length is "less-than" one-tenth of the wavelength then you probably don't need to operate with a terminator. Even at slightly longer trace lengths you could terminate with a series combination of a few tens of pF and (say) 50 ohms. This avoids the problem of a 50 ohm terminator directly across a logic line. For different circuits the "rules" are more stringent for instance, a photodiode amplifier might have a 3dB bandwidth of 1 GHz (wavelength = 0.3 m) and one tenth would be 30 mm - a totally disastrous trace length on the input to a photodiode amplifier and also the inductance of the line would cause all sorts of hidden surprises when trying to get it working. So rules change depending on what you're trying to do. 


So I'm making a distinction here between robust digital (or analogue) transmission, sensitive/feeble circuits like photodiode amplifiers and I'll use your 6.5 GHz UWB as an example - it may have had broad tuning across a couple of GHz but if you were trying to make a linear amplifier from the kHz to GHz range you will hit problems on trace length inductance resonating with parasitic transistor capacitance and sometimes you have to put resistors in very small tracks just avoid a circuit self-oscillating. With my "radio head" on what you can achieve at really high frequencies (but limited bandwidth) means you can utilize parasitics to your advantage but not so across a really wide bandwidth from DC to several GHz. That's how it tends to pan-out for me anyway.


Prevention of long traces radiating can be done with balanced traces - the far field is  zero because the two EM fields cancel out (when done properly). Using striplines is a technique and doesn't itself stop a signal radiating. Coax does of course and so does balanced stripline.
Micro output impedance isn't as relevant as you think in a lot of examples - say it is 10 ohms at 100 MHz - your output goes down a 50 ohm stripline (or coax) and providing the termination at the receiving end is adequate, reflections are minimized. I know at college they say your output needs to be impedance controlled but in reality it doesn't.