Splinterland Brawls are a guild event where you can earn merits and SPS. I believe that Brawls are a great way of earning SPS and merits even if you don't have a powerful modern deck, and that is why I try to participate in every single brawl. It doesn't matter how well you do, but winning fights does improve the rewards slightly.
Brawl Result
I chose the gold foil modern Brawl and got 7 wins. This time, there were 5 free wins from guilds that didn't submit people. My rewards for the brawl were 3,476 merits and 43.995 SPS in the 4-star category. My guild ranked 7th place out of the 10 guilds there. I choose to take the gold foil modern brawl because my guild usually doesn't fill all its spots, and I prefer to give my other guild members a chance at the brawls. I did decent, but my guild didn't do so great either.
Taking a look at the results, I only submitted 4 matches to get over 40 SPS. This is why I believe that Brawls are so rewarding in terms of SPS. This amount of SPS is far higher than what I am currently getting in my Modern ranked battles at the Bronze/Silver League for winning around 20 fights per season (around 4-5 SPS). Even when my guild didn't do so great, that 23 SPS was quite good. Looking at the log, it seems like there is a brawl every 5-6 days so submitting the matches every single time doesn't seem so bad.
On top of that, there is also merits that can be spent on Gladius Cases. These cases give gladiator cards, and while they are not as useful due to some summoners being removed from modern, these cards do increase your win rate in Brawls.
What do you need to do to get Brawl rewards?
In order to get brawl rewards, you must do the following two things: 1) enter a brawl fray (during the prep phase), and 2) submit all fights into your matches (during the combat phase). You don't have to do anything during the reward phase. For the first thing, it is possible for a guild officer to put players into empty frays. So even if you forget, you might still be able to take part in that, and this will also increase the points for your guild because the free wins do add to your guild's score. If they don't put you in and you forget step 1, then you can't participate in step 2. Step 2 must be done by yourself. If you lose the match or surrender, I believe it still counts as you submitting your match.
Conclusion
Overall, I find that Brawls are very rewarding, and there isn't too much work to get those rewards. Compared to the amount of fights you have to do for Modern, the amount of SPS you earn is far higher when you don't have as strong a deck.
Do you participate in Guild Brawls, and do you find them great for rewards?