I would say 2010 and 2014 were disasters for the Democrats, but not 2012 and perhaps 2016 if one get past the White House, pretty darn close. I haven't delved into state legislative races outside of a glance every once in a long while. In 2016 Clinton did win the popular vote by close to 3 million, the Democrats did pick up 2 senate seats and 6 house seats. Governors wise Missouri, Vermont and New Hampshire switched to the Republicans, North Carolina went Democratic. 2016 certainly wasn't a wipe out although the lost of the presidency might make it seem so. But 2016 basically showed both parties at relatively even strength. What this portrays to 2018 is anyone's guess at the moment. But if Trump remains as unpopular as he is, I would expect big Democratic gains in the House, the numbers are against the Democrats in the senate. As for governors, you're the expert there.
History shows that in the first mid term the party that holds the white house loses seats in congress, sometimes rather big as happened in 1954, 1966, 1970, 1978, 1982, 1990, 1994 and 2010. 9-11 pushed Bush's first mid term back to his second mid term where he lost 33 seats. But every president since Eisenhower through today lost seats in congress in their first mid term with the exception of the second Bush. His first mid term loses pushed back 4 years due to 9-11.
Right now the two parties are fairly even in both being disliked by over 50% of all Americans and in the strength of their base. The strength of each party is at 30% each of the total electorate with independents making up the remaining 40%. But since the inauguration independents lean Democrat has increased from being even with independents lean Republican on election to an advantage of 18-12 over independents lean Republican with 10% of the 40% being what I term pure or true independents with no leans. So today the numbers point to a good Democratic 2018 election. But that is so far off and these numbers so dynamic, they mean little to nothing today. Just give us an idea where things stand. But a look at Trump's job approval numbers will tell you that.