I have been running a solid flywheel for about 25,000 miles and the downsides are more vibration and general harshness, the cv joints and gearbox have a harder time ( less cushioning effect than with a dmf).
It is advisable to use a lower gear (ie 2nd instead of 3rd) to keep the revs higher as the low down torque,( the main advantage of a jtd!), is what gives the drive train a hard time, well that is my theory, anyway!
The upsides . . .a slightly faster revving engine, less inertia as the weight is more central than a dmf.
When the clutch goes, it is almost certain you will only need a clutch kit!
If you shop around it is possible to find dmf and clutch kits for a similar price to smf and clutch conversion kits. As that is the case, when my clutch gives up the ghost, I'll be going back to dmf, probably!
Dmf will cope with a mild remap ok, too, as long as the clutch is in good condition!
Hope that helps, a little.