I've been with Nexcess for a few weeks now, following an inability to run Magento 2 on my Site5 server. I've run in to a few major issues with the installation and configuration of M2, but these guys have had an answer to everything I've asked - even resolving a problem that was stumping the internet at large! I can't recommend this company enough - planning on switching my reseller setup from S5 to them next :)