a) tax reasons ...
A camcorder (or VHS recorder...) has a diff. = higher tax rate; devices which are not 'camcorders' = defined by length of video, cost a few cent less on import ... Why the DSLR makers don't accept these few bucks - I don't know. For me as a customer, an unlimited 450€ is 'better' than a restricted 419€ .... 😝
(Not so long ago, you could 'hack' those Euro-specific-limitations. Or, some buy vDSLR for that reason in US or Asia.. )
b) AVCHD vs. mov/mp4:
The AVCHD structure contains the mts = video PLUS 'other' data; in this case i.e. the 'advice': "stitch mts1 with mts2" to one single video. Many 'smart people', who strip-off the AVCHD-structure and orphanize the mts, notice in some NLEs 'gaps', flashing frames etc. = the 'stitching' doesn't work correctly. As a keyword in that context: if the codec uses GOP, ideally the GOP is closed at the end of #1. If not, the meta-infos in those 'other' AVCHD-files tell the NLE to proceed. Many 'green flash issues' are base upon this.
c) Camcorder are designed for long recordings
e.g. my tiny Sony NEX5 supports fullHD video - but gets incredible hot when doing so beyond 20-30min of continous recording. My Pana is meant for long videos: hardware is optimized, enough CPU-power included. So, there's no hardware hickup, when the SDcard reports "single file full!", there's enough 'room' to prepare the switch from file#1 to file#2. (could be a cam's Ram thing.. beyond me).
plus 'Image': vDSLR have the image of 'pro' = used for scenic/theatrical projects = short 'takes', no '2h a piece'. A camcorder is more a 'Daddy films daughters ballet'-thingie.
d) exFat
In theory, such DSLR could use exFat (or NTFS) as card-format; why this isn't done, is beyond my knowledge. Patents & licence fees, I assume...
e) due to 4k res, 4:4:4 encoding, 60p/120p/240p fps etc etc the sheer amount of data makes many cam-makers to decide NOT using AVCHD, but simple mov/mp4 as wrappers; or their own, as Sony mxf, or Reds .r3d, which ignore those histroic limitations.
trivia:
sanDisk announced lately a 512GB SDcard = half a terabyte! on a 'stamp'.... 138 chunks of 4GB..... 😁
