How accurate is the 130 plus?

I use the Edge 130 Plus for MTB and overall it does what I need. I use it often for navigation (I design my own GPX tracks starting from Trailforks trails and Strava segments) and it's quite good, considering the small display.

It has a few shortcomings, see here.

I have the regular 130, only because I bought it before the Plus was introduced.  As 120CCPM says, it really does all I need for MTB.  Although Trailforks maps would be nice, it isn't necessary.  I've successfully navigated many new trails at locations I'd never been to before.  Without the 130, I'd have needed a guide.  Sometimes you have to miss a turn by 50 foot to be sure, but that's perfectly acceptable to be able to ride at a new place.

I no longer use Strava segments, as I only care about the downhill, and the lawyers won't let them work on the device.  And I don't want to give Strava anymore money.  But, for the short time I used them, they worked.  One flaw, is that at popular locations, the 130 will automatically start tracking a popular segment.  If another segment you've starred starts soon after, tough luck, it won't show another segment until the first one has finished.  This is probably similar across all Edge devices.  I'd have preferred if Garmin didn't automatically activate segments that haven't been starred.  I don't know of anyway to prevent "popular" segments from showing up.

The main glitches with my 130 are poor cold weather battery life and an elevation graph that sometimes works, sometimes doesn't.  Both issues have been addressed numerous times in the forum, but pretty much ignored by Garmin.  I solved the battery problem with a handlebar mounted lipstick battery which I need to just charge once for the entire winter.  Fortunately the device allows "through charging", i.e., charging while using.

If you can put up with a few typical Garmin let downs (as you're likely already used to), then I'd highly recommend.  The screen is crisp and easy to read.  The optional remote is a must, allows page control without moving your hand off the bar.  I'd also highly recommend a speed sensor, and manually input the tire circumference for it, which makes for fabulous accuracy.  "Auto calibrate" just doesn't work with MTB.  Roll out the tire next to a tape measure while pushing down to simulate some rider weight.  Go three revolutions and divide by three for the answer, which makes the mileage dead-nut accurate.