I've heard there are concerns with this type of equipment not having isolation transformers, especially with "hi-fi" type tube power amps. Obvioulsy there are very high plate voltages, which without an isolation traffo can potentially travel up your guitar cable. Not sure if that would pertain to what you're talking about.
You heard wrong. They dont have isolation transformers, but no guitar amp does. Isolation transformers are not used for that anyway.
Your input (the guitar cable) gets noplace close to the the plate voltages... except in each tube.
That has often been a source of debate. What happens if a tube shorts from grid to plate? The guitar is connected through a resistor to the grid... but on some amps, its connected directly, and the resistors normally used are 68k, not huge. They're there for isolation, should you decide to use both inputs at once... ie, two guitars at one input. Some amps (very few) use a capacitor input at the first grid. There are problems associated with that, and generally, the tone and dynamics suffer.
However, on a turntable, film strip projector, old movie projector (all sources of good low wattage amps), they run them pretty much like a guitar amp. If the tube shorts grid to plate, then you got a real bad (but real short lived) problem. In all the dealings I've ever had, I've not once seen it. Ever. There will no doubt be stories of ... that guy I knew from Kalamazoo had a cousin, who knew the brother-in-law of a gal at work, who once shocked himself silly! Very few actual reported incidences of it, and the situation is no better, or no worse today, and all the manufacturers still use the same circuit, even in todays lawsuit happy society.
So... to wrap it up... the turntable amps are no worse, and no better than a guitar amp in terms of safety.