The Selts were armed with particle cannons and shield crackers, the latter they developed into web breakers.
Particle cannons are basically slightly less powerful disruptors that fire twice per turn. Their overall effectiveness is a bit better than disruptors.
The shield cracker only damages shields. It has no effect on the ship itself. It was designed to allow the Seltorians to just knock down the shields of a criminal/dissident and then board and capture the vessel (their ships are equipped with large transporter arrays and lots of boarding parties) rather than destroy it. The Seltorians secretly developed the web breaker from shield cracker tech. Against Tholians this weapon is very effective.
Since the designs of their ships are for capturing though and not destroying an opponent they are at a bit of a disadvantage in battle against most other races. This is where the idea of their weapons sucking comes from. They're just over specialized for standard combat tactics.
Selt ships are also heavily FA arc oriented in their weaponry and not very maneuverable. They were designed to chase down their adversary. Standard anti-Selt tactics are to get behind them. Since there weren't any seeking weapons used in the Tholian home galaxy (they had evolved beyond that weapons wise) they also have poor point defenses.
The Seltorians were the "police" force of the Tholian empire. They were actually created by the Tholians. Much like the subject races of the Dominion in DS9. Unlike DS9 though the Seltorians did not regard the Tholians as their gods and resented them. The story of the Tholians and Seltorians closely mimics Frankenstein, where the creator is destroyed by his own creation.
Once the Klingons realized that the Seltorian force that arrived in our galaxy was merely a scout force and was going to wait for reinforcements from their home galaxy before actually trying to eliminate the Tholians (A trip that would take 200 years) they weren't very happy with their new allies. The Klingons had no intention of letting the Tholians stay in the Holdfast for 200 years. When they did the math on the Seltorian's prolific breeding, they decided that the Seltorians were a threat to the Klingon Empire and called in the "exterminator" themselves. Between the ISC and their "allies" the Klingons, the Seltorians had a short history in our galaxy.