There are a lot of Democratic Candidates Running
I’ve never seen anything correlating the number of Democratic candidates running and Democratic success. Darrell Issa is more likely to lose if he has seven Democrats running against him instead of two. It's certainly helpful if they challenge more Republicans than less, but that really wasn't the problem for Democrats in the past. Democrats had candidates in all but one of the Clinton districts a Republican congressional candidate won.
Fundraising is going well especially with grassroots progressive groups
Fundraising has never been a problem for Democrats and they seem to taut their fundraising advantage every cycle. There's evidence that a candidate needs to spend above a certain threshold for the voters to take him or her seriously, but after that's achieved spending isn't always an indicator of success. Democrats heavily outspent Republicans in the GA-6 race and they didn't win that. They didn't lose CA-49 in 2016 for lack of spending. They spent a lot.
Republican retirements are high
This is seen as an indicator for two reasons. First, these congressmen must know more than we do and that's why they're retiring. Second, open seats are much easier to flip. The problem here is that Republican retirements aren't all that high. Here are the numbers since 2006:
2006: 18R, 9D
2008: 27R, 6D
2010: 20R, 17D
2012: 19R, 22D
2014: 25R, 16D
2016: 25R, 18D
2018: 17R, 8D
What we see here is that Republicans have had higher retirement numbers in good Republican years and lower ones in good Democratic years. This year could be an all time high for Republicans but only 10 of the 17 Republican retirees are running for another office. in 2014 only 11 of the 25 retirees ran for another office. Generally, congressmen running for another office announce earlier to get an earlier start on campaigning. So right now 7 Republicans are actually retiring, compared to 14 in 2014. There were 20 retirements in 2016. I see no reason to think we'll get closer to the 20 than the 14 and Republicans actually did well with the 20 retirements last year.
While you may be hearing Republicans in competitive districts are retiring, that doesn't appear to be the case. Only 2 of the Republicans in a Clinton district is retiring. Donald Trump won Charlie Dent's PA-15 and Dave Trott's MI-11. Those could be competitive districts but they aren't going too be competitive solely on Donald Trump's unpopularity.
The Generic Ballot is Good for Democrats
This is true and the Democrats are better off with a good generic ballot than a bad one. The Democrats led for most of the 2014 and 2016 cycles, however, and lost the congressional vote each time. Polling seems to have had a tougher time finding Republican voters than it has in the fast. So this could be good for Democrats but it might not be.
2018 should be a good year for Democrats but I see nothing yet to tell me it definitely will be.