I was the tau player, first of all we have to bear in mind my list was not meant to be any good

it was meant to have suits in it. So I can't really say anything about the power of the list or units, I didn't have any particular concerns that anything was severely OP, not on the basis of one game with a suboptimal list. I was disappointed with the result but then I couldn't make any rolls, lost strategy on turns 2 and 3 (1 too?), and then had a senior moment in turn 3 going after the wrong objective. On a better day this game could have been a draw.
It's true I didn't like the spotter rule, but this is mostly because I found it incredibly confusing, especially taken together with the support craft rule. I think between us we managed to confuse ourselves about where units needed to be in order to see/hide from the overlord and colossus! I do also think the spotter range is excessive when you consider the similar Tau rule is a defining feature of that list and is only 30cm. They are a force multiplier, and at these ranges they can spot for extreme range weapons whilst being relatively safe. I don't think this does good things for the play style.
I'm not sure if the board size really makes it any harder to stay out of reach of the colossus, I mean is there anywhere on a 6x4 board where you can avoid a 120cm/180cm indirect barrage when the unit still gets to move?
I did in fact prioritise the overlord and gyros, I just didn't have anything that could kill them. Normally I would expect to have more options but my list is just crap

Losing strategy turns 2 and 3 stopped me being able to deal with them, quite simply.
Bear in mind though that these units start on overwatch which prevents you being able to target them at the beginning with anything except long range artillery, particularly overlords with many 75cm AP/AT shots. As soon I was able I did shoot them (IIRC with the overlord this was very early on and think even broke it, but it rallied). After that when the broadsides died I had nothing that could kill it. It does seem a very good unit for its points because:
- it is resilient
- it puts out silly firepower
- it has 75cm range
- it can spot for a 90cm MW barrage
It's all very well it being "so good that it gets targeted by the enemy" but that doesn't stop it being good. If I am shooting several formations at your overlord, that means I am throwing them into your guns and not shooting at other things. Overlords will get targeted yes. So do markerlight units of the enemy knows what they are doing. Your job as a tau player is to make killing them expensive and frustrating; I see no reason why it should be any different for squats.
Stubborn I didn't really mind, again it is a little inelegant and frankly unnecessary but I'm not bothered. As I mentioned on the 1.5 thread, it did feel strange on the thunderfires. On a unit that can't move its a straight buff, normally the big downside of immobile units is they can't escape when assaulted so are automatically destroyed. Same when breaking after supporting an assault. For squats that doesn't happen unless you have the move to CC them, otherwise you need to use another activation to get rid of them. An odd side effect for sure, I would like to see what happens when there are 4 of these formations on the board.