Summary
Allen is a pretty efficient scorer, finishing in the top 25th percentile in Points per Shot Attempt each of the last two seasons, according to Cleaning the Glass. That's mainly due to his above-average three-point shooting (40% for his career), and the fact that the majority of his field goal attempts come from behind the arc.

While Allen does have some off-the-dribble ability, he doesn't get to the rim often and isn't someone that you want having a high usage for your offense. Allen was assisted on the vast majority of his buckets, so he's largely dependent on others creating shots for him.

On defense, Allen has good, but not great, size and athleticism for the wing, and he generally plays hard. He can guard either guard position but would likely struggle to defend forwards against most teams.

I think there's a chance that Allen thrives playing in Milwaukee, as he should get plenty of open looks from three and could be a solid defender that doesn't have to be given the tough defensive assignments.

Allen will already be 26 when he hits free agency in 2022, so while this may limit his upside and any expectations of major improvement, the team that signs him will likely have Allen for the remainder of his prime.
 
Cap Considerations
Allen will be a restricted free agent with a $12.2 million cap hold, $5.8 million Qualifying Offer, and full Bird rights. I wouldn't expect Allen's QO to come into play, but I wouldn't completely rule it out as I could see a scenario where his market is dry as a restricted free agent and the Bucks play some hardball given their luxury tax concerns.

From other teams, Allen will be looking for a 4-year, $40+ million offer. I think that an offer sheet for the full Non-Taxpayer or less is an option for Allen, so teams without cap space could be interested. In addition, adding a 15% trade bonus, a player option, and some advances on his salary could make it more difficult for Milwaukee to match.