I like the idea of making the fatty able to build on the move, assuming it's technically achievable. If the built units could also automatically follow the fatty around (like an assist order) that would be incredibly nice. It would help set it apart from the megalith and it would make the ability more useful since sitting still is more costly for the fatty, due to it's low speed, and more dangerous, due to it's low health. It's also an "interesting" buff that needs player attention and micro to take advantage of, and not just a boring stat increase.
Even as things are now, I think this ability is underused, both for the mega and the fatty.
Just imagine how much better a fatty push could be if you could set it to repeat build t3 engineers once it got completed, without having to stop: by the time it reaches the front you might have enough engie escorts to consolidate all the ground you gain, rebuild all dead mexes along the way, reclaim everything and build firebases strong enough to dissuade even direct fire experimentals from attacking. Shield spam could easily tip the scales vs an arty base or air snipe.
We usually see this kind of tactical flexibility only once SACUs start hitting the field, because people are reluctant to mix t3 engies into their lategame spam. I think that's because it's too tedious to manage their rally point and they end up being too far from the fight to achieve their potential most of the time.
Thematically, the fatty is supposed to be a mobile factory and it's just a shame and a missed opportunity that it's so clunky to use it as such right now.