Generally, I would not call them safe. There are several issues, the main one being that the maximum rated plate voltage for the el84 is 300V. Many 6V6 amps, even the low power ones, run at 400 to 450 volts. Some (like the Princeton reverb) don't even have screen resistors.
Now, it is possible to run high plate voltages on an el84, but you really need to use sane voltages on the screen (like one-half b+) and hefty screen resistors (like 1k; not the tiny 470R that Fender used to use). That is, you need to design the output circuit with the plate dissipation and voltage ratings of that particular tube in mind. Traynor used to do this with some very clever designs (super custom special anyone?) but they were the exception. Well, ok music man did this kind of thing as well.
So, in general, no. Not safe. Which is annoying because you can't find 6V6 tubes right now