reliability

How reliable is arduino uno?
How reliable is an Opto-Isolated 8 Channel Relay Board?

What sort of answer do you want?
A MTBF number?
This is a meaningless question without a specific setup. Even then you can only find out by testing several hundred in a test chamber for about a year.

How reliable is arduino uno?
How reliable is an Opto-Isolated 8 Channel Relay Board?

Very reliable, but I will not connect one to my heart lung machine.

rs99tx:
How reliable is arduino uno?
How reliable is an Opto-Isolated 8 Channel Relay Board?

Please explain what you want to do. Nobody can understand why you are asking this.

rs99tx:
How reliable is arduino uno?
How reliable is an Opto-Isolated 8 Channel Relay Board?

Fairly reliable, if you get the right ones and use them correctly.

Hope that answers your question.

rs99tx:
How reliable is arduino uno?
How reliable is an Opto-Isolated 8 Channel Relay Board?

Uno - pretty reliable, but I wouldn't use it for mission critical systems.

Relay board - reliable, but if you're using mechanical relays, they will have a finite life that depends to some extent on how often you switch them.

Ok I guess that was a vague question. How about this…
Is arduino uno suitable as a controller to run a device 24/7 monitoring sensors and preforming tasks or is it more suited to be manually powered up to run the tasks.
Example A
Look at water level sensor and add as needed -> Filter for 15min every 12hrs.
Example B
Power Up -> Add water as needed -> Filter for 15min turn off.

Relays – Is this the correct way to control 24vac sprinkler valves the will cycle a few hundred times a week.
Option A (Relay)
5V 8-Channel Relay interface board, and each one needs 15-20mA Driver Current Equiped with high-current relay, AC250V 10A ; DC30V 10A
Option B (SSRelay)
FR-4 Fiber glass PCB (Double Layer); Size: 155 x 55 x 24 mm (approximately 6.1 x 2.17 x 0.95 inch)
Input control signal voltage: 0V - 0.5V Low stage (SSR is OFF), 0.5V - 2.5V (unknown state), 2.5V - 20V High state (SSR is ON); SSR Output (each channel); Load voltage range: 75 to 264VAC (50/60Hz); Load current: 0.1 to 2 AMP

What you are looking at doing is reasonable for the Arduino.
There are many here that have 24/7 applications running.

Solid Sate devices are vary reliable, but keep in mind your circuitry may be subjected weird software bugs, power failures, static discharges, moisture or other unforeseen conditions.

I always add auto/manual and disable circuitry (if possible) to projects in the event of a hardware failure, example, if a transistor shorts or a relay welds shut.

Try to include fallback circuitry to prevent serious damage.