One of the advantages claimed for K track is that by using the Flex track you can build large radius sweeping curves.
Presumably if you have a section on a C track layout where a larger radius than R5 would be desirable you could just use one or more Flex tracks for the curve with K-C transition straights where the curve starts and ends. This seems obvious, are there any disadvantages to such a solution?

The benefits of K flex in an otherwise C layout are that you can make curves and routings that you cannot with the c track system. The gentle bends and weaves on a straight to large radius curves.. and size it to fit in one piece vs a mix of C track to match the 360mm grid. The only issue, as mentioned, is matching the transitions up. K track sits lower, so you need to raise it, the rail itself is different "code" sizing. The k track rail is taller than the C-track, so if you build your own transition and directly attach the K track to the C track after bringing up the K track so the top of the rails are at the same height, you will see the bottom of the steel rails don't align, so you need those transitioning rail joiners to connect things up there. (plus the electrical connections you have to make. That does allow transitioning from C->K and K->C in a more customized space than the straight transition pieces from Märklin.

My planned layout still includes the idea of using flex k-track on some longer opens spaces of non-turnout running, such as from one tunnel to the next. With K you then need to ballast it to match the C track (whose ballast you can colorize or amend), so there is more work to make it all look more integrated, but thats part of the modeler fun.