Both shows have engineers as the leading men. But also, both shows have Asians (Benson Fong) on the show.

I am not seeing Asians on other family themed shows of the era like FKB or LITB.

Was this because of the engineering themes?
Am I the only one who notices this?

They both seem to explore a lot of cultures. Family Affair went to Spain and I think Italy. My Three Sons went all over the place.

Benson Fong was a noticeable presence on M3S, and we got a good look at Asian/Chinese culture in some of those episodes. He only appeared in a few episodes, but it was throughout the series (checking other sources, seven times 1963-1969).

Both Fedderson-produced shows had cast intros. welcoming viewers to the show. Most of the time these are cut from later copies, since they often included sponsor plugs in the original airings.

Speaking of Far East themes, we had the Family Affair episode on Japanese culture, with the little bonsai tree from Mr. Osaki, and mention of his flowering plum trees (and park trees being his friends). There was also a hilarious episode where Nigel French botches the language, when trying to speak to the little Asian girl who's a friend of Buffy & Jody.

Bill Davis' civil engineering work took him to the South American continent.

These series shared music, which is no surprise; when urban/industrial building exteriors were shown we had the fast paced one:
da da da, da-da da da da DA...

Lots of actors appeared on both series, like Sherry Alberoni, Joan Vohs, John Willams, Veronica Cartwright, Sebastian Cabot, and William Boyett.