The Thunder and Arbalest are very different weapons, one heavy and old while the other is a relatively new light weapon. However, the one thing that they both have in common is that many players have complained that they are both underpowered when compared to other weapons, sometimes even of different classes. It is important to look at both weapons separately and compare them to their counterparts.
The Thunder used to be one of the best heavy weapons in the game. Unfortunately, over time its damage has been reduced significantly through several nerfs. Even though it has a range of 500 metres, its effective range is far less (250 metres and under) due to bullet spread, with minimal damage occurring at longer ranges. Being a kinetic weapon, it does double damage to physical shields, and is therefore an effective shield breaker. Even when at ranges of 400 metres, due to bullet spread, all of its projectiles will hit an Ancile at that distance, further increasing its usefulness in this role.
However, its damage is amongst the lowest for a close-range heavy weapon, despite its shield-breaking ability. When compared to the Ember, it is completely outclassed. The Ember has a higher effective range, bypasses all shields, has a shorter reload and does nearly 50,000 more damage overall. Even the Redeemer does substantially more damage than the Thunder. The Thunder's situation is even more dire when you compare it to the recently buffed Storm, its medium counterpart. The Storm does nearly as much damage as the Thunder, with the same shield-breaking ability, even though it is a medium weapon. The only advantage the Thunder offers over its heavy rivals is its cost-effectiveness, being available for only 580,000 silver.
Secondly, we have the Arbalest. Being one of the newest additions of the game, this weapon costs 10,000 components. Even though Arbalest components are cheaper and are easier to purchase than most other component types (e.g Kumiho components), it is still much more expensive than most other light weapons and despite that, is notoriously underpowered. Offering great synergy with the Trebuchet, it has a much shorter charging period and does less damage, but has the same mechanism as its heavy counterpart. Being an energy weapon, it bypasses energy shields such as the Ancile. When it fires, damage is instantly dealt to the target hit.
No matter how you look at it, the Arbalest is chronically underpowered. Even the Gekko, its main rival, despite it being easier to get (750 Gold), does much more damage over a slightly longer perod of time. This is an embarrassing weakness for the Arbalest, which was designed and released as an alternative to the Gekko, which was often used with the Trebuchet and Zeus. The Gekko is also an energy weapon, bypassing Anciles as well. Those two weapons are the only long-range sniper weapons in the Light category. Despite component weapons usually being favoured by high-league weapons, usually due to their superiority, many snipers using the Trebuchet continue to use the Gekko, as it is cheaper (arguably) and does substantially more damage. Perhaps suprisingly, the Arbalest is ironically even more underpowered than the Thunder (most new additions to the game are overpowered, this is one of the first to be underpowered).
Overall, both the Thunder and Arbalest are in need of a significant damage buff in order to compete more effectively with their rivals; However in the case of the Thunder, it remains a potent and mighty weapon to be respected, and is still used by many players for its cost-effectiveness and decent damage.