If the decision to go with an agent is made, I think trust has to be the biggest factor, along with that comes morals, work ethic and desire. As I'm sure you've seen, some recent players are signing with guys because they're 'flashy' and give off an entertaining persona. This should be business and personal relationship, not one built off of the perception of who might be fun or who can't sell themselves so they have to throw cash out to become appealing. I understand taking a loss leader and paying for a player's training as that is now expected, but agents offering any other inducements just shows me they aren't confident in themselves, and if you're not confident in your brand/product/abilities then how can you be expected to go head to head with an NFL executive for your client and do a great job? Some guys have negotiated big contracts and years of experience, but also many lawsuits or scandals involving former clients, which can be easily found. As you mentioned, you're also seeing some players signing with agents who have 100 clients. Sure they have experience and have been around the game, but do you think they're really taking their time and negotiating hard deals that long into their careers, when they have so many other clients to handle?
As I'm weighing my future job paths I have those same fears, do I go with a small agency or even alone where there's less security but I have more of a voice, or a mega firm where my job is likely safe, but I'm just a number and have little say? I think football contract and marketing negotiations under one roof is beneficial, and can allow the player to work with the same person/ few people rather than another whole team. But I think finances should be separate and act as a form of checks and balances.
I think its all up to the agents personality, his/her background and work ethic. Everybody starts somewhere, Shaq showed up to UM as a freshman with no experience and more experienced guys ahead of him, but he had the mentality and work ethic that he would not be denied, and ultimately he was named the starter and the rest is history. If I was in the NFL, that's the personality trait I would look for when determining who is going to negotiate on my behalf. And I don't see it as bashing, everyone is entitled to their opinions and its a big decision to make. There are some very bad apples in the game that unfortunately give the entire industry a bad rap.
Edit: A thing I missed, if Lamar Jackson stays without an agent; is he going to have the balls for lack of a better term or confidence to tell his GM "no" or whatever it may be when negotiating his offset language? Some guys might but I'd think the vast majority don't. His lawyer can sift through his contract and make any suggestion, but Lamar is the one that has to make the demands to his employer, then go back and forth and listen to his employer tell him why he isn't good enough or worth changing the contract language. Then he has to play for said employer for years and constantly be reminded of what this man truly thinks about him and his worth.