The engine wiring harness and throttle body wiring are the greatest bugbear with the M104's.
However if changed the M104 becomes a much better long term proposition than the M103 with CIS. CIS has too many expensive parts (some N.L.A.) plus finicky distributor plugs and leads.

Head gasket on M104 is an issue but also on the M103 to a lesser extent.

I prefer the first gear start, better power and economy of the last series M104's.