They limit loadout options. The impact cannons, storm cannons and shards now only fire side/back, and there's no direct counterpart to them for the heavy offensive turrets. You can no longer mount a forward-firing 1.0 hybrid cannon, a kinetic hull buster, or shotgun, as examples.

Single heavy-slotted cruisers have limited flexibility. Now you must make the very hard choice of mounting only a single forward firing gun: A mortar is better for attacking at longer ranges, tachyons are better for chasing gunboats and consistent hit rate, a missile is very useful in group battles and for surprise head-on attacks. You can no longer have situational forwarding firing guns for such ships: you're forced to use your back guns, reposition yourself, or hit the nearest massive solar object like a planet to escape your painful existence.

Inviabilizes mounting missiles in single heavy-slotted cruisers. I can no longer feasibly mount a missile because I completely lose heavy projectiles to my front, which is an enormous disadvantage. With version 5.0 360 guns, I could cover all sides with heavy projectiles to alleviate the problem, but not anymore.