I would love to offer some different point of view...maybe it is not that much about song qualities or staging but something about statistics and numbers...As far as I'm concerned, I think that so far there hasn't been situation when more songs from first half of SF qualified than from second half...here are situations from previous years since introduction of two semi-finals:
2017 SF1: 5-5
2017 SF2: 5-5
2016 SF1: 5-5
2016 SF2: 5-5
2015 SF1: 4-6
2015 SF2: 3-7
2014 SF1: 5-5
2014 SF2: 4-6
2013 SF1: 5-5
2013 SF2: 4-6
2012 SF1: 4-6
2012 SF2: 4-6
2011 SF1: 4-6
2011 SF2: 5-5
2010 SF1: 4-6
2010 SF2: 5-5
2009 SF1: 3-7*
2009 SF2: 3-7*
2008 SF1: 3-7*
2008 SF2: 5-5*
*Televote only + jury invitation wildcard
As we can see, there is not situation when more songs from first half went to final, always in best scenario it was 5-5 or it has been prevailing to second half. Since jury-televote 50-50 voting (in different ways of points calculation - as we know, it was changing through years), but it is noticeable that songs from first half became more competitive! Especially in last two years when points from both, jury and televote are just summed up, outcome is completely equal! In previous years, when televote would have advantage in case of tie, we can notice that in most cases songs from second half would be favored. Anyhow, why I'm speaking all this?
When looking currently on odds, we can see some strongest SF favorites are allocated in first half! Acutally, top 4 by current betting odds of SF1 preform in first half! Those are: Israel, Estonia, Bulgaria and Czech Republic! And among favorites to qualify (currently top 10) are Belgium (for long time strong favorite) and Belarus while very strong esc nation that has never failed to qualify, Azerbaijan, is not even in top 10 but at 11th position! Some nations have strong, outstanding hopes and stagings like Albania and Lithuania! Maybe even Iceland can shock everybody out of nowhere?
Anyhow, based on history statistical trends of favoring second half and taking into account great songs in first half of SF1, I think there will be some shocking surprises in first half and some hard to believe non qualifiers! Who it will be? I do not have idea...maybe I would say Iceland, Albania, Belarus, Lithuania and Czech Republic/Azerbaijan/Estonia but any, but nominally any of those songs could qualify!!
Hmm...but who knows, maybe history will change and for first time there will be more songs from first half to go to final than from second half?