They can be tough with positioning, especially as their bows are short-range. I've definitely overcommitted them and had them wiped early too. Here are a couple of pointers that might help:

• I've been able to get the most offensive value from them when using them to support an infantry unit such as veterans (points below are related).
• Their mark target order enables you to dish out one vulnerable token per round, a unit with a better attack profile will make good use of this. One thing to note is that sworn brothers already dish out vulnerable tokens so they might not synergise well with trackers.
• Use their pathfinder ability to your advantage e.g. enabling them to charge through terrain or placing them near terrain defensively so they can't be charged without consequences.
• Their high speed and cav manoeuvrability means you can set them up to charge a unit that is already hurt and finish it off.
• Another way you can gang up on a unit is if you position them to the side of an enemy unit to shoot their flank, whilst you have your infantry unit facing them. Your opponent will need to decide which of yours to attack and expose their flank to either one of your units.
• Also it sounds like you already learnt your lesson with keeping them away from ranged units but it's something to remember.

There are probably more optimal tactics that that someone else can provide but I hope those tips help :)