New tractors of ANY brand can and sometimes do have issues that take some effort to solve. Interchanging suspected parts that control functions like starting, such as a ECMs don't always fall off a dealer's shelf. If ANY manufacturer is having a problem with something as complex as a ECM, it may be present in all or some of the units both on the tractors and replacement units. This does NOT indicate a 'lemon' tractor, or a poor quality brand either.
The fact that something had to 'be done to the battery' too, could point to issues with the tractor that were not found until the ECM was replaced, possibly with a second new unit. Dealers and manufacturers don't always disclose what they have done to fix something that they set right under warranty. Their obligation is to do the repair, which is not necessarily a full disclosure situation, as part of their remedy.
If the OP experiences further issues report them here and to your selling dealer. And look at who, mostly Deere people, in this thread, is slamming your brand choice, its relays, - NOT the problem, etc.:confused2:
On further thought, its possible the battery was the cause of the entire problem from the start.
The ECM could have been replaced by the dealer, and when going to load the tractor it became evident the issue was NOT the ECM, but something more obvious- hence the battery was likely changed out, and problem solved.