These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Macekre: The English dubbed version of the original Japanese dialogue is not nearly the worst offender, but the differences are definitely noticeable.
BCI Eclipse's DVD releases of the series, thanks to having the uncut episodes, show at which points the show was or wasn't dubbed - a Japanese TV episode is a couple minutes longer than an American episode, so where there were undubbed moments, the dialogue just switches back to Japanese, and the subtitles come on. Whether it's incredibly distracting or just really distracting depends on whether or not you know why it's happening.
Nightmare Fuel: Dada and the Mummy will have you looking behind your back when you're alone in an office building...
Nightmare Retardant: ...until you imagine Muramatsu - not an especially big guy - tackling Dada to the ground and not even losing any momentum in the process.
Special Effect Failure: Occasionally. It was made in 1966, and not everything's going to look good after 50 years.
Visual Effects of Awesome: Put it this way - it's 1966, and most of the crazy shit you see in a typical episode would have looked good in a theatrically-released film. This was a half-hour kids' television show, filmed in color at a time when there were barely enough black-and-white T Vs to watch it on. They could've fudged things here and there, but didn't, and wow does their effort show even nearly 60 years after the fact.