We're playing an Al Qadim campaign (arabic setting with Genies and flying carpets instead of dragons) for one and a half year now every character starting at level 1. Today most of the group is slightly above level 10 and the game is not that diversified like it was in the beginning.
First I've to say we like tactical combat a lot. So combat maneuvers like Charge, Weapon Bind, Powerstrike, Press&Melee are in heavy use. Thus we had a very high body count which means the group started once with 6 mages and one rogue. Most of the mages died by stupid mistakes for example got eaten by giant lizards or were forced to marry an high level Gen and were added to his harem. Players learned a lot and built their new characters in a tactical way which can be described as powergaming. HARP allows that if the GM doesn't start adding hard restrictions.
So today some players have their third of fourth character and they tend to be very specialized and combine spells in an effective manner which is the main problem and the feeling of not being balanced anymore. So there are two mages left. One is kind of an Elementalist who specialized in Air Form, Elemental Aura, Thunderclap and Counterspell. A very deadly combination
The other one pushed his sleep spell so hard that any kind of NPC who ever get to roll against needs to have 96+ regardless how much RRs he's got if not immune to sleep. In addition he is good at Fire Nerves, Counterspell and Dispell Magic. Of course both mages assist the party with Bladeturns. The rest of the party is a Shadowblade with outstanding ambush skills using "Shadowport" often, a Harper who maxed "Charm" and being a good fighter, a Cleric able to bless, heal and resurrect the party and being the fighter with the highest melee damage value and finally the rogue being the second sneaky character. No one is dual classed because it's a loss of effectivity.
In HARP you can specialize in a single skill a lot and as NicholasHMCaldwe said there are talents for mages which are in heavy use for spells that need to be scaled up often like Counterspell. The possibility to cast very powerful spells at low levels is a feature of HARP which is missing in D&D or RM but sets the system aside and gets it incomparable. I really love the idea of being able to scale spells but it has a downside.
We're using default rules and no modified scaling. A mage with Eloquence Talent is way more effective than the occasional +10 a fighter gets. Like bunny mentions heroes don't collect only experience points but magical items as well so they get boosted by such things like PP adders, items that grant bonus to skills, reduce penalties, etc. jasonbrisbane is right: the mage doesn't have to cast the spell at the highest possible effect but the RR to be made are so high that it's no fun for a GM to create immunities or special characters/environments all the time.
As GM you can't use foes out of the box to get an entertaining session were combat is still thrilling for everyone because the fighters get outperformed by our mages so heavily. At least it's the combination of their specialized skills and teamplay. They really know how make a haul with most non magical encounters. So we end up mostly with encounters that are only beatable because of the mages although player character death is not uncommon it starts to be a stressful job for me as GM and I'm looking forward to end the campaign soon. It's not the spells I mentioned above. I saw other combinations working effectively as well. When the GM ends up creating foes for the mages only there is something wrong.
I got the impression if you use the additional books and don't add restrictions HARP can be unbalanced in mid to high lever character setting. I mean what will they do if they reach level 18+ ?
