With a mid to late 70's unit, I'm guessing the issue for some people is that sourcing original components in the event of a replacement being needed (some of which haven't been made for decades) is going to be challenging at the very least and potentially expensive.
On a positive note though, there's only has 8 VFETs to worry about (the TA-8650 and TA-N7B both had 12 VFETs).
I've heard one of these - incredibly smooth for SS when played through speakers with an 8 ohm impedance load, but sounded a tad less lush when switched to the 6 ohm taps. A TAN-5650 with its beefier trannies should be a better option in such circumstances.
As a further endorsement, Nelson Pass revived the VFET technology via the SIT outputs used in his First Watt power amp.