I agree with all, one of the problems between H323 and SIP (or SIP-T)is the complexity of H323.
But I think that the main problem is the interconnection between a VoIP network and a TDM network.
In this case you have two ways :
1- interconnection under ISDN protocol (ITU Q931)
2- interconnection under SS7 protocol (ITU Q7xx)
For case 2 the protocol used as ISUP V3 is really complex with a large number of information elements in messages. The difficulty is to translate these information elements between a ISUP message and a H323 or SIP message for ensure a correct treatment of the call.
It’s necessary to have a complete mapping between the protocol messages, and nobody knows why but H323 is not fully compliant with ISUP (same standard organism – ITU – but no concertation between the teams). At contrary SIP is FULLY compliant with ISUP V3 and that’s the reason it’s a better choice. Think that when you use H323 you add a large delay for the treatment and the translation of the information elements of each message receive from TDM network. With SIP you don’t have this problem.