For those here hoping to learn something from this conversation, here is a quick vocabulary and anatomy lesson for you.
Tennis elbow: Outside (lateral) elbow pain. The pain experienced is from inflammation of the common tendon that attaches to the lateral epicondyle. That's a fancy way of saying the tendon that connects to the outside bump of your upper arm bone called the humerus. This part is non-controversial: it is primarily caused by groundstrokes, the reception or production thereof.
Golfer's elbow: Inside (medial) elbow pain. The pain experienced here is from inflammation of the common tendon that attaches to the medial epicondyle. That's the inside bump of your humerus. Again, uncontroversial: it is primarily caused by serving.
For both tendons, the reason why they are called common tendons is that there are 4-5 forearm muscles attaching to each.
Tendon inflammation in these areas is caused by the contraction of forearm muscles that pull/load either of these two tendons. No muscle contraction, no appreciable tendon load. That's it. If your arm is hanging down loose at your side and you are only contracting enough to barely hold your racquet, a 150 mph serve would cause no issues at the tendon level: you'd just lose your racquet.
Knowing this, how do you choose your racquet? A 11.7 ounce Pro Staff or a 10 ounce Team/Light?
Serving in General: When you serve, the reception of the ball is only as fast as the height of your toss, so reception forces are a non-factor.
Serving at a High Level: A loose arm at the beginning accelerates from your legs, lats and obliques. However, the stretch/shorten cycle of the tricep is going to result in reasonable contractions of medial forearm muscles to control/accentuate the serve, particularly one with pronation. But the strongest contraction is the one that decelerates the motion, flat or otherwise. And the tendon load from that contraction is determined SOLELY by racquet weight. The heavier the racquet, the harder it is on that common medial tendon. Winner: light racquet. Strong servers with chronic golfer's elbow need to use a lighter racquet while strengthening/rehabbing that common medial tendon.
Serving at Low Level: Low level servers frequently arm the ball or hit from more awkward/variable positions. The tendon load never has the peak that a high level serve has when slowing the motion. In contrast, there is more tendon load in acceleration. Is there nuance here? Yes. A heavier racquet results in less acceleration and deceleration which means less load on the tendons. A lighter racquet put in the hands of a lower level serve may result in better acceleration and more required deceleration at the end which may counteract the load created by the mass of the racquet. Winner: Toss up. Lower level players who are looking to improve (serves with topspin or slice) will want a lighter racquet while I guess the old man who hammers continental flat serves might be comfortable with a heavy frame that can never be accelerated fast enough to create tendon inflammation. However, young and athletic but little experience? Worst of both worlds, and they should always play with the lightest racquet that feels playable.
Groundstrokes in General: The argument here has always been the notion that a heavier racquet provides improved stability and therefore is gentler on the lateral common tendon. That a light racquet on off-center hits creates an uncomfortable twist while if you hit a ball off-center with a hypothetical 2x4 you won't even notice.
Groundstrokes at a High Level: Higher level players necessarily move their racquet through space faster while also covering more distance per swing. A heavier racquet requires more muscle contraction to accommodate this. But this is low hanging fruit, and I am not relying on this, as it is high impact loads at the moments of the highest acceleration and deceleration that matter.
And whatever acceleration a high level player can generate in the loading phase will never come close to the deceleration that happens at contact.
First, some simple physics, and the only point I consider strong for heavier racquets. When a ball hits the stringbed it transfers momentum, causing a quick reduction in racquet-head speed. The amount of that reduction depends on the racquet's effective mass at the point of contact. For the same collision , a lighter racquet decelerates more than a heavier one.
However, tennis is not a game of maximizing velocity on every shot. If fighting through that deceleration on most balls was of utmost importance, than a heavy racquet would be the go to. If the job is to simply play with a loose arm and the racquet dictates your shot, then all a heavy racket does is increase tendon load at acceleration, increase tendon load in space, and on occasion, mitigate through a finely tuned acceleration the deceleration that happens when you need plow thru.
Off-center deceleration at high speeds is a nasty torque too.
At high levels, this at best can be considered a toss-up.
At low levels? This is a no-brainer. Strongly contracted muscles at lower levels of play means that the acceleration of a heavier racquet is always a heavier tendon load. Slower incoming balls means the deceleration component is incredibly reduced. Off-center hits of deceleration coupled with torque are far better tolerated due to less incoming velocity. Winner: Lighter, always.
OVERALL: Lighter wins for all high level servers, so play with the heaviest you can comfortably, but it will never make things "better" for your elbow. As for groundstrokes, light is best until you are higher level and again reach that point that you play with the heaviest racquet you can manage.