Ugh. Just completing a migration from WSS 3.0 (SP 2007 vintage) to SP 2013 Foundation. One of the nastiest, most confounding experiences of my career and I've been working on ERP systems for almost 30 years.
To be fair, the only real problem was with their Search service, which is not really a migration issue. But it just didn't work, been troubleshooting for almost 3 weeks now. Just today finally had a breakthrough, appears it was Windows Server 2012 permission issues deep in .NET implementation which prevented key services from running that Search depends on. And I mean permissions for local users group and Local System that should have been there. Or were there and that stupid UAC stuff was screwing it up.
Hopefully I'm retired before I have to mess with SP 2016 or beyond. The sad thing is, MS brought very little new business value in SP 2013. It runs faster and works without being in Quirks mode in IE, meaning it now can work in Chrome and other browsers. But beyond that, hard to find anything useful unless you are into that My Sites stuff. Be interesting to see what SP 2016 says it can do.