I had switched to Betterment for about 18 months at one point and then realized I was paying them (yes a small fee, but in the MMM mindset, all money wasted is wasted) for very little after observing the process, so I moved everything to Vanguard. Betterment used about 10-13 holdings to diversify and I also saw no reason to deal with that as the simple 2-3 fund portfolio is shown to perform similarly and that portfolio mix in Betterment has higher management fees in general or they were using a Vanguard fund and then still adding their 0.25% - 0.15% fee on top of that. At the time it was hard to see the exact portfolio they would build until I used them. As I watched over 18 months and really analyzed what was going on I got more and more irritated that I had switched. At small amounts that is not much but it starts to get into thousands of dollars a year around $400K and over 20-30+ years into retirement I felt this was not worth it.
I will say moving the funds from Betterment to ANYTHING is a royal PITA. Their systems to get in and run are great. Getting out they literally send me an Excel spreadsheet so that Vanguard could figure out my cost basis. It took about 3 months to clean things up as Vanguard has to hand enter these into their system because the file could not be imported. I made contributions weekly, so I had at least 75 or so purchases on 10-13 holdings, so about 1,000 entries. After that experience I would never use Betterment again. It was a disaster, and I was paying for that disaster. I have now been with Vanguard again for about two years and love it.
Rebalancing does not need to happen as often as Betterment likes to do it (this was one of the things that drew me to them because I have in the past forgotten to look at things for 4-5 years and been off a bit), once or twice a year is good enough.
So my vote is save yourself money and just go to Vanguard. Be prepared for a bit of a headache unless they fixed things (oh they heard about the process from me and I escalated it all the way to the top, so perhaps they fixed that). Vanguard was amazed at how the could not do a simple like-kind transfer with cost basis information.
I'll be happy to talk more if you'd like.
ETA: Better initial explanation