I've built 3 amps with the 5T (50VA)series, and will use more. They are just fine. I have seen a couple cases where people used these undersized for the demand, like a pair of 6V6s, and wondered why the votages were low, but even then, they used them sucessfully. I used one for apair of 6AQ5 which are close to same spec as 6V6, and it is about at max, voltage gets pulled down if biased hotter.
They seem to handle extra load OK up to a point. I personally still would avoid using them at over their rated specs, and bump up to a 100VA as needed.
In another case, someone built a micro, and the 5T did not have enough load to draw it down to the spec voltage, so B+ was too high. They provide charts showing voltage at differeing currents, so size them accordingly.
One drawback for circumstances where you want to use a rectifier tube, some are not center tapped, so bridge rectifier is in order. A hybrid bridge with rectifier tube can be accomplished, but they are not generally plug and play for a 5Y3 etc. Some do have two secondary coils that can be run in series or parallel with CT. You need to understand the coil configuration in the context of what you want to do.
Advantages: Toroids are quieter. The price is right.