According to my short experience in pipeline projects, there are not only advantages in the use of fully welded valves because in case of problems such as blocking ball due to lack of valve maintenance , or damage of the seats or even of the ball, the repair operation may be heavy and very expensive: it can require repatriation of the valve to the manufacturer shop.
On the other side, when using split body valves, a leak from body flanges can occur. Consequently, one must have a continued and vigorous monitoring of facilities. Repair is not so difficult but still requires testing of the valve after replacing seals.
In my humble opinion, the use of welded body valves will be limited to underground valves and difficult access facilties. Where it is easy to carry out checks it is more interesting to use split body.
Regards.