I’ve never agreed with the argument that doing it sectionally is important as you need to be in the ‘correct mindset’ for each section and that doing them in order can screw you up. I think the greatest time for sectionally is that you can quite easily break down your timing. For most people section 1 definitely takes a significantly longer all together compared to something such as section 2. Instead of doing it in order and saying to your self ‘2 mins for each section 1 question, 1:30 for Section 3 and 1:30 for section 2’, you can simply allocate, say, 80 minutes to JUST do section 1 questions.
This means you may need to only check your watch every few minutes compared to almost every question. It also means that when you realise you are going over time, you can decide to ‘quick guess’ the rest of the questions for that specific section and be sure that you will still have adequate time for other sections.
Ultimately, in my opinion, there is no clear disadvantage in doing the UMAT in order provided that you have a solid strategy in mind to manage your time. It is mentioned constantly that time management is one of the most important skills for the UMAT. I simply believe that sectionally just makes it easier to manage time in comparison to in order. But if you are certain you can manage your time go for it!