TF2 at its core is a game about specialists and generalists. That is at its core, the game and the mechanics operates the best when it's played like that. If you go into any high-level pub, with players who know what they're doing (they may not have amazing DM or huge technical skills, but assume they're using they're using their mics and have some semblence of co-ordination) there is a natural class balance that flourishes.
That class balance heavily favours classes like the soldier, scout, demo, heavy and medic, anyone who isn't completely pants-on-head retarded will see these classes are simply better at doing 90% of the things you need accomplish to win the game. Yes, there are a few snipers, engineers, spies but all those classes collectively make up a tiny percentage of the numbers, that's why they're specialists. Forcing a team to have as many specialists in the game as there are generalists will just make the game worse. Both to play and to spectate.
Now Valve has tried to address this issue by putting in unlocks that make it more viable to play full time spy, sniper, engineer but I think we can all agree that these unlocks are not fun to play against, especially when they were first introduced.
A lot of times when people debate weapons it's always the statistical balance that gets argued, but no one tries to argue about the weapon being NOT fun to play against. Why is that? Are we forgetting that this is a game and however balanced an item may be, if it's not fun to play with or against then it should be immediately banned?
We play TF2 for ourselves, we don't have some sort of huge spectator crowd behind us, we don't have huge sponsors behind us, what makes TF2 tick is the players. And if something isn't fun to play against, it should be banned.
Hence why people hate stuff like the mini-sentry, jaratee, wrangler, pre-nerf GRU-sandvich combo, pre-nerf enforcer/spycicle/DR combo, etc, etc. The list goes on. They aren't fun to play against, I think that should be a valid criticism and grounds for a ban.
A lot of these unlocks are also non-skill indexed. What I mean by that is that the skill required to overcome the unlock is not equal to the skill required to deploy it. Case in point, the wrangler, any idiot can equip it and take it out and swap between that and hitting the sentry. You've just gained yourself a 600hp sentry that can heal by hitting it with a wrench (building a sentry can hardly be classed as difficult). Now what do you have to do to kill it? Well firstly you most likely have to build an uber, then you have to co-ordinate with your demo or heavy and both of you need to stay alive for at least 40 seconds. Now those aren't really hard things to do, but they are much harder than what the engy has to do.
I mean all of the not-fun unlocks are like that, it takes more co-ordination to take down than it does to use.
Then there's the fact that Highlander is a defensively orientated game-mode. If you look at a 6's team with default classes, it excels at movement and sustained damage output. A highlander team excels at holding and stagnant play. You're trading having 2 scouts and 2 soldiers for having classes like the pyro, engineer and heavy, all who are excellent at holding points than getting points. And you also gain 2 unreliable pick classes in the sniper and spy.
Unfun unlocks, non-skill indexed items, defensively orientated team makeup, broken class composition.
I honestly feel like Highlander is being propped up by the terrible people who can ONLY play engineer, spy, heavy and pryo. If it weren't for these bads HL would die right off. No one good at the game would actually try to main engineer.