The honest reality is porting all libraries will take time. But not nearly as much time as Teensy 3.0/3.1 took, and many will "just work".
The M0+ chip we're using has a very similar peripheral set at Teensy 3.1. Most even have the exact same registers. Some libraries using Teensy 3.1 registers will work as-is, or with very little work. This isn't going to be anything like 2 years ago, starting fresh on a brand new platform with absolutely no software support.
Two years ago, many of the important libraries had AVR timer registers hard-coded. Many of them used timers only to run an interrupt routine at some predictable rate. I ported all of those to use IntervalTimer. So all those libraries will "just work" with a working IntervalTimer in the core library.
Also, since 2 years ago, the entire Arduino world has started changing. Especially Adafruit (who are responsible for a lot of the modern Arduino libraries) have adopted much more hardware neutral approaches. Others have too. Some people are still hard-coding AVR stuff, but it's not nearly as terrible as it once was. Teensy 3.1's avr_emulation.h has also come a long way since the early days!
But not everything is going to be perfectly smooth. Some libraries, like IRremote, are going to take some significant work. Some code will probably hard fault due to unaligned access (which works on M4, but faults on M0+).
The best I can tell you is, unlike pretty much all the other Arduino-compatible board makers, including Arduino themselves, I absolutely am committed to making nearly all 400-some widely used Arduino libraries work. I believe I have a pretty well established trend of not necessarily having everything smooth and perfect at release, but a pretty responsive process of truly investigating compatibility problems and fixing/porting libraries.
I hope you'll have realistic expectations, and most of all, I hope you'll post reproducible bug reports (eg, the "forum rule"). The vast majority of the time I spend on bugs is figuring out how to reproduce them. Once I get it happening on my workbench, I usually come up with a fix very quickly. Please, help me out if you can, with detailed reproducible bug reports!