I can speak specifically for buy vs build from the build-it guy perspective.
I almost always build because it's usually cheaper to do so and when it's not, I end up with a customized solution tailored specifically for my needs and taste. I can tell you that the summary below is probably not what most would expect because of it.
Built (Analysis) - I have a few that function very similar to an MfT. Some were made specifically for assembly and some for fabrication (primary difference is if the top is sacrificial/replaceable or not). All have machines hole surfaces for clamping with the Festool clamps and that, in and of itself, is the best feature by far - infinite capabilities. Some were made with aluminum extrusions for rigidity and to allow side clamping like the MFT and some have wood sides with and without holes for clamps. Did they end up cheaper? Yes, I believe the cost per square foot is lower for a built MFT. One of mine was completely built out of Baltic birch ply - maybe $80 for the top and saw horses for legs. Cheap. All of mine (except the giant) are portable and can follow me in my MDX if I could find a reason.
Becoming a DIY MFT3 - difference is the miter assembly and rail components. There are many examples out there that people have built that function very similarly to the one standard on the MFT3 but, other than those who use a table saw miter assembly, most don't have the angle capability and, instead, tend to look to widgetry for cutting angles (such as dogs and mathematical magic using multi-points). Most methods will require your holes to be spot on for repeatability and accuracy. Those who kinda get it buy the Festool parts and use them on their homemade table but the miter assembly and rail parts are expensive separately and make build a silly argument at that point.
Buying an MFT3 - since I built all of mine before buying I can tell you this with confidence - you unpack it and use it to do what you had originally intended to work on. You confined to the factory size but you can buy a second one if you need bigger or make cheaper extension tables to just extend. If you have Kapex wings they're compatible. Maybe at some point you buy a replacement top but get to your main project quickly you will.
Today (Reflection) - since acquiring my fest tool MFT3 I now use it exclusively for machining (Tracksaw, jigsaw, domino) activities. My large table is now used exclusively for assembly and my smaller tables are used exclusively for assembly and smaller fabrication tasks like pocket hole drilling and sanding. If I could only have one eyed want the Festool MFT3.
Summary - if I had to do it all over again I'd answer only one question before I made my decision:
Build - Is the MFT3 a project or is it a tool? If I like mental gymnastics, enjoy the process of copycat fabrication, have size needs that exceed the stock footprint, have the needed capabilities and equipment/tools or are willing to build/but then, budget doesn't support buy, and/or am content with maybe not having every bell and whistle.
Buy - If time is a factor, it's a tool to be used in other projects and isn't itself the project, the factory format is all you need, and/or it's just gotta be Festool.